Glossary

A1C is a blood test showing average blood sugar over the past two to three months, reported as the percentage of hemoglobin carrying sugar.

Amortization is paying off a debt through regular instalments that each cover interest plus part of the principal, so the balance reaches zero by the end of the term.

An annuity is a series of equal payments made at regular intervals, such as monthly deposits into savings or monthly income paid out from a lump sum.

An API (application programming interface) is a defined way for one piece of software to ask another to do something or share data.

APR (annual percentage rate) is the yearly cost of borrowing, expressed as a percentage, that includes the interest rate plus certain fees.

APY (annual percentage yield) is the real yearly return on savings once compounding is included.

ASCII is a 7-bit character encoding that assigns numbers 0 to 127 to English letters, digits, punctuation and control codes.

ASCII art builds pictures and large lettering out of ordinary text characters.

An ASN is a unique number assigned to a network, or group of networks, that is run under a single routing policy on the internet.

Aspect ratio is the proportion of an image or screen's width to its height, written as two numbers such as 16:9.

AVIF is an image format based on the AV1 video codec that gives very small files at a given quality and supports transparency, animation and HDR.

AWG, the American Wire Gauge, is a numbering system for wire thickness in which a smaller number means a thicker wire that can carry more current.

Bandwidth is the maximum amount of data a connection can carry per second, usually quoted in megabits per second (Mbps).

A barcode is a pattern of bars and gaps that encodes a number or text so a scanner can read it quickly.

Base32 is an encoding that represents binary data using 32 safe characters, the letters A to Z and the digits 2 to 7.

Base64 is an encoding that writes any binary data using only 64 safe text characters (A-Z, a-z, 0-9, + and /), so it can pass through systems built for text.

HTTP basic authentication sends a username and password in a request header, joined with a colon and Base64-encoded.

Battery capacity is the charge a battery holds, in milliamp-hours (mAh), or the energy it stores, in watt-hours (Wh = volts × amp-hours).

Binary is the base-2 number system, using only the digits 0 and 1, which computers use because electronic circuits are either off or on.

The binomial distribution gives the probability of a number of successes in a fixed number of independent yes/no trials with the same success chance.

Blood alcohol content is the percentage of alcohol in the blood; the Widmark formula estimates it from drinks, body weight and time.

BMI is body weight in kilograms divided by height in metres squared, a screening number for whether weight is in a healthy range.

BMR is the energy your body burns at complete rest to keep you alive, estimated from weight, height, age and sex.

Body fat percentage is the share of your total weight that is fat, estimated from tape measurements or skinfolds.

Body surface area is the total skin area of the body in square metres, used to scale drug doses and some physiological measures.

A bond is a loan to a government or company that pays regular interest (the coupon) and returns the face value on a set maturity date.

BPM, beats per minute, is the tempo of music: how many beats occur in a minute.

The break-even point is the sales volume at which total revenue equals total costs, so profit is zero.

Brightness shifts every pixel lighter or darker, contrast spreads the light and dark values apart, and grayscale removes colour, keeping only luminance.

A broken link is a hyperlink whose target no longer loads, usually because the page was moved or deleted, and returns an error such as 404.

Browser artifacts are the traces a web browser leaves on disk: history, downloads, cookies, cache, saved logins and session data.

A brute-force attack tries every possible password or key, or a huge list of likely ones, until one works.

A BTU (British thermal unit) is a unit of heat energy; BTU per hour measures the heating or cooling power of equipment.

A budget is a plan for spending and saving income; the 50/30/20 rule allots 50% to needs, 30% to wants and 20% to savings and debt repayment.

A byte is a unit of digital information made of 8 bits, able to hold 256 different values from 0 to 255.

The Caesar cipher is a simple substitution cipher that shifts each letter a fixed number of places along the alphabet.

CAGR is the constant yearly growth rate that would take a value from its starting amount to its ending amount over a period.

A calorie, as used for food, is a kilocalorie (kcal): the energy to raise 1 kg of water by about 1 °C, equal to 4.184 kilojoules.

A canonical URL is the preferred address of a page when several URLs show the same content, declared with `<link rel="canonical">`.

The capitalization rate is a rental property's yearly net operating income divided by its price, a quick measure of its income yield.

A capital gain is the profit from selling an asset, such as shares or property, for more than you paid for it.

A CDN (content delivery network) is a network of servers around the world that stores copies of a site's files and serves them from the location nearest each visitor.

A certificate of deposit is a savings product that pays a fixed interest rate if you leave the money untouched for a set term.

The chain of custody is the documented record of who handled a piece of evidence, when, where and why, from collection to presentation.

A checksum is a small value calculated from a block of data and used to detect accidental errors when the data is stored or sent.

A chi-square test compares observed counts with the counts expected under a hypothesis to see whether the gap is more than chance.

The cholesterol ratio is total cholesterol divided by HDL ("good") cholesterol; a lower ratio generally indicates lower heart-disease risk.

CIDR (Classless Inter-Domain Routing) is the notation that writes an IP network as an address plus a prefix length, such as 192.168.1.0/24.

A codec (coder-decoder) is the method used to compress and decompress audio or video, such as H.264, H.265, AV1, AAC or MP3.

Colour blindness, or colour vision deficiency, is a reduced ability to tell certain colours apart, most often red from green.

Colour contrast is the difference in luminance between text and its background; WCAG expresses it as a ratio from 1:1 to 21:1.

A colour palette is a small set of colours chosen to work together in a design.

A combination is a selection of items where order does not matter; `C(n, k) = n! / (k! (n − k)!)` counts the ways to choose k from n.

A commission is pay calculated as a percentage of the sales a person makes, either flat or in tiers that change with volume.

The compile timestamp is a date stored in a Windows executable's header, in seconds since 1970, recording when it was linked.

A complex number has a real part and an imaginary part, written a + bi, where i is defined by i² = −1.

Compound interest is interest calculated on the original amount plus all the interest already added, so growth builds on itself.

A confidence interval is a range around a sample estimate that would contain the true value in a stated share (such as 95%) of repeated samples.

A container format, such as MP4, MKV or WAV, wraps one or more encoded streams (video, audio, subtitles) and their metadata into one file.

A Content Security Policy is an HTTP header that tells the browser which sources of scripts, styles, images and other content a page may load.

A cookie is a small piece of data a website stores in your browser and sends back with later requests, used to remember logins and preferences.

CORS (Cross-Origin Resource Sharing) is the browser mechanism that lets a server allow web pages from other origins to read its responses.

A credit score is a number, such as 300 to 850 for FICO scores in the US, that summarises how reliably you have repaid debts and is used by lenders to set approval and rates.

Cron is the scheduler on Unix-like systems that runs commands automatically at times set by a five-field schedule expression.

Cropping, rotating and flipping change an image's framing and orientation without altering the colours of its pixels.

Cross-site scripting (XSS) is a vulnerability in which an attacker gets a website to deliver malicious script to other users' browsers, where it runs with the site's privileges.

CSRF (cross-site request forgery) is an attack that tricks a logged-in user's browser into sending an unwanted request to a site where they are authenticated.

CSV (comma-separated values) is a plain-text format for tables, with one row per line and values separated by commas.

A CVE (Common Vulnerabilities and Exposures) is a unique identifier, such as CVE-2021-44228, assigned to a publicly disclosed security vulnerability.

CVSS (Common Vulnerability Scoring System) is the standard way to rate how severe a vulnerability is, on a scale from 0.0 to 10.0.

A data breach is an incident in which personal or confidential data is accessed, disclosed or lost without authorisation.

A dead pixel is a screen pixel that stays permanently dark, while a stuck pixel is locked on one colour.

The debt avalanche is a payoff strategy that puts all extra money on the debt with the highest interest rate first, which minimises total interest.

Debt consolidation replaces several debts with a single new loan, ideally at a lower interest rate or with a simpler payment.

The debt snowball is a payoff strategy that puts extra money on the smallest balance first, for quick wins, then rolls each freed-up payment onto the next.

The debt-to-income ratio (DTI) is your monthly debt payments divided by your gross monthly income, which lenders use to judge whether you can afford more credit.

A decibel is a logarithmic unit that compares two levels of sound, signal or power; in digital audio, level is measured in dBFS, with 0 as the maximum.

The default gateway is the router a device sends traffic to when the destination is outside its own subnet.

Density is mass per unit volume, `ρ = m / V`; it tells you how tightly matter is packed.

Depreciation is the loss in value of an asset over time through wear, age or obsolescence, or the accounting method of spreading its cost across its useful life.

A derivative measures how fast a function changes at a point; it is the slope of the curve there.

The dew point is the temperature to which air must cool for water vapour to condense into dew; the higher it is, the muggier the air feels.

DHCP (Dynamic Host Configuration Protocol) automatically gives devices an IP address and the other settings they need to join a network.

A digital certificate is a signed file that links a public key to an identity, such as a domain name, so others can trust that key.

A digital signature is a value computed from a message and a private key that proves who created it and that it has not been changed.

A dividend is a payment a company makes to its shareholders out of its profits, usually every quarter.

DKIM (DomainKeys Identified Mail) adds a digital signature to outgoing email that receivers verify with a public key published in DNS.

DMARC is a DNS policy that tells receivers what to do with email that fails SPF and DKIM alignment, and where to send reports about it.

DNS (Domain Name System) is the internet's directory that translates domain names like example.com into IP addresses and holds other records about a domain.

A DNSBL is a DNS-based list of IP addresses known for sending spam or abuse, which mail servers query to decide whether to accept a message.

Dog years convert a dog's calendar age into a rough human-equivalent age; the old rule of multiplying by 7 has been replaced by better models.

Dollar-cost averaging means investing a fixed amount at regular intervals regardless of the price, so you buy more shares when prices are low and fewer when high.

A domain name is the human-readable address of a website or service, such as example.com, which DNS translates into an IP address.

A down payment is the part of a purchase price you pay upfront from your own money, with a loan covering the rest.

The electric network frequency is the tiny, constantly changing variation around 50 or 60 Hz in the mains supply, which leaves a hum in many recordings.

An electronic signature is any electronic mark, such as a typed or drawn name, that shows intent to sign a document.

An email address has a local part, an at sign and a domain, such as `[email protected]`, and identifies a mailbox.

An emergency fund is cash set aside for unexpected costs such as job loss or repairs, commonly three to six months of essential spending.

Encryption is the process of scrambling data with a key so that only someone with the right key can read it.

In security, entropy measures how unpredictable something is, such as a password or key, usually expressed in bits.

Equity is the part of an asset you truly own: its current value minus what you still owe against it.

Error level analysis (ELA) is a technique that recompresses a JPEG image at a known quality and maps how much each area changes, to help spot parts that were edited or pasted in.

An escape sequence is a backslash followed by a character that stands for something that cannot be typed directly in a string, such as a newline or a quotation mark.

Escrow is an arrangement in which a neutral third party holds money or documents until the conditions of a transaction are met.

Estate tax is a tax on the value of a person's property at death, usually applying only above a large exemption amount set by law.

EXIF (Exchangeable Image File Format) is metadata stored inside photos that records the camera, settings, date and often the GPS location.

The expense ratio is the yearly fee a fund charges, expressed as a percentage of the money you have invested in it.

An exponent says how many times to multiply a base by itself: in `2³` the base is 2, the exponent is 3, and the value is 8.

The factorial of n, written n!, is the product of all positive whole numbers up to n, and counts the ways to order n distinct items.

A favicon is the small icon a browser shows in a tab, bookmark or history list to identify a website.

An FHA loan is a US mortgage insured by the Federal Housing Administration that accepts smaller down payments and lower credit scores in exchange for mortgage insurance.

File carving is recovering files from raw storage by searching for their start and end signatures, without relying on the file system's directory records.

FIRE is a savings approach that aims to build a portfolio large enough to live off its returns, typically 25 times yearly spending, long before the usual retirement age.

A firewall is a system that allows or blocks network traffic according to rules about addresses, ports and protocols.

The Flesch Reading Ease score rates how easy English text is to read from its average sentence length and syllables per word; higher is easier.

A forensic image is an exact, bit-for-bit copy of a storage device or file system that investigators analyse instead of the original.

A fraction represents a part of a whole as a numerator over a denominator, such as 3/8.

Frame rate is how many video frames are shown per second (fps), such as 24, 25, 30 or 60.

A freelance hourly rate is the price per billable hour needed to cover your target income, expenses, taxes and unpaid time.

Fuel economy is how far a vehicle travels on a unit of fuel (miles per gallon, km per litre) or how much fuel it uses over a distance (litres per 100 km).

GDP is the total value of goods and services produced in a country in a period, the standard measure of the size of an economy.

A geotag is geographic location data, such as latitude and longitude, attached to a photo, video or post.

Gestational age is how far along a pregnancy is, counted in weeks and days from the first day of the last menstrual period.

GFR is the volume of blood the kidneys filter per minute, estimated from a creatinine blood test, age and sex.

Git is a distributed version control system that records snapshots of a project's files so you can track changes, go back and collaborate.

A golf score differential compares a round to the difficulty of the course, so scores from different courses can be compared and a handicap computed.

GPA (grade point average) is the credit-weighted average of course grades converted to points, commonly on a 4.0 scale.

The greatest common factor of two numbers is the largest whole number that divides both without a remainder.

A half-life is the time it takes for half of a quantity to decay or disappear, whether a radioactive isotope or a drug in the body.

A hash is the fixed-length fingerprint a hash function computes from any input; the same input always gives the same hash, and the input cannot practically be recovered from it.

The heat index is the "feels like" temperature that combines air temperature and relative humidity to show how hot it seems to the body.

HEIC is the file type for images in the HEIF format, which stores photos compressed with the HEVC codec; it is the default photo format on iPhones.

Height is written in feet and inches in some countries and in centimetres in most others; one inch is exactly 2.54 centimetres.

A HELOC (home equity line of credit) is a revolving credit line secured by your home, with a draw period followed by a repayment period.

A hertz is one cycle per second, the unit of frequency; for sound it sets the pitch.

Hexadecimal (hex) is the base-16 number system, using digits 0-9 and letters A-F, widely used as a compact way to write binary data.

HMAC (hash-based message authentication code) is a hash computed with a secret key, which proves a message came from someone holding the key and was not altered.

Horsepower is a unit of power; one mechanical horsepower is 33,000 foot-pounds of work per minute, about 745.7 watts.

The hreflang attribute tells search engines which language and region each version of a page is for, so users see the right one.

HSL describes colour as hue (position on the colour wheel in degrees), saturation (intensity) and lightness, which makes tints and shades easy to create.

An HTML entity is a code such as `&amp;` or `&#169;` that stands for a character that is reserved in HTML or hard to type.

HTTP (Hypertext Transfer Protocol) is the protocol browsers and servers use to request and send web pages, files and data.

HTTPS is HTTP sent over an encrypted TLS connection, which keeps traffic private and confirms the identity of the server.

iCalendar is the standard text format for calendar events and invitations, stored in .ics files that calendar apps import.

The ideal gas law relates pressure, volume, amount and temperature of a gas: `PV = nRT`.

Image compression makes an image file smaller, either exactly (lossless) or by discarding detail (lossy).

The import table of an executable lists the library functions the program calls, such as Windows API functions.

An indicator of compromise (IOC) is a piece of observable evidence, such as a file hash, IP address or domain, that suggests a system has been breached.

Inflation is the general rise in prices over time, which reduces what each unit of money can buy.

The internal rate of return (IRR) is the yearly discount rate at which the present value of an investment's cash flows adds up to zero, a single percentage summarising its profitability.

An invoice is a document a seller sends to a buyer listing what was supplied, the amount owed, the due date and how to pay.

An IP address is the number that identifies a device on a network so that data can be sent to it.

IPv6 is the newer version of the Internet Protocol, using 128-bit addresses to replace the nearly exhausted 32-bit IPv4 space.

An IRA (individual retirement account) is a US personal retirement account with tax advantages that you open yourself, outside any employer plan.

ISO 8601 is the international standard for writing dates and times unambiguously, in the order year, month, day, such as 2026-10-03T14:30:05Z.

The IV drip rate is how fast an intravenous fluid runs, in drops per minute: `volume (mL) × drop factor (drops/mL) / time (minutes)`.

JPEG is a lossy image format designed for photographs that makes small files by discarding detail the eye is least likely to notice.

JSON (JavaScript Object Notation) is a lightweight text format for structured data, built from objects, arrays, strings, numbers, true, false and null.

JSON-LD is a way to embed structured data in a web page as JSON, using schema.org types, so search engines understand what the page contains.

JSONPath is a query language for picking values out of a JSON document, the way XPath does for XML.

A JWT (JSON Web Token) is a compact, signed token made of three Base64URL parts that carries claims such as who a user is and when the token expires.

A keyframe, or I-frame, is a video frame stored as a complete picture, while the frames between keyframes store only the changes.

A kilowatt-hour is the energy used by a 1,000-watt device running for one hour, the unit on electricity bills.

Latency is the delay between sending a request and starting to receive the response, usually measured in milliseconds.

A leap year has 366 days, with 29 February added, to keep the calendar aligned with the Earth's orbit.

The least common multiple of two numbers is the smallest positive number that both divide into evenly.

Linear regression fits the straight line that best predicts one variable from another by minimising the squared vertical distances to the points.

A load balancer is a system that spreads incoming requests across several servers so no single one is overwhelmed.

The loan-to-value ratio (LTV) is the loan amount divided by the value of the asset securing it, a key measure of lender risk.

Localhost is the standard name for "this computer": a loopback address that sends network traffic back to your own machine.

A logarithm answers "what power do I raise the base to in order to get this number?": `log_b(x) = y` means `b^y = x`.

Long division is the step-by-step method for dividing large numbers, giving a quotient and a remainder.

Lorem ipsum is dummy Latin-like text used to fill page layouts and designs so that reviewers focus on the design rather than the words.

A MAC address is the 48-bit hardware identifier of a network interface, used to deliver data within a local network.

Macronutrients are the nutrients needed in large amounts: protein and carbohydrate (4 kcal per gram each) and fat (9 kcal per gram).

Magic bytes, or a file signature, are the fixed bytes at the start of a file that identify its true format regardless of its name or extension.

Malware is any software designed to harm a device or steal from its user, including viruses, worms, ransomware, spyware and trojans.

In trading, margin is money borrowed from a broker to open a larger position than your own cash would allow, secured by your account equity.

The marginal tax rate is the rate applied to the next dollar of income, as opposed to the average rate across all your income.

Markdown is a lightweight way to write formatted text using plain-text symbols, such as # for headings and ** for bold, that converts to HTML.

Markup is the amount added to a cost to reach the selling price, expressed as a percentage of the cost: `(price − cost) / cost`.

Mass is the amount of matter in an object; weight is the force gravity exerts on that mass, `W = m × g`.

A matrix is a rectangular grid of numbers used to represent linear equations, transformations and data.

Mean arterial pressure is the average blood pressure during one heartbeat cycle, `diastolic + (systolic − diastolic) / 3`.

The mean is the sum divided by the count, the median is the middle value when sorted, and the mode is the most frequent value.

An image macro is a picture with a caption, usually bold white text with a black outline at the top and bottom, that spreads online as a meme.

The title tag and meta description are the page title and summary that search engines often show as the blue headline and the snippet beneath it.

Metadata is data about data: the hidden or secondary information a file carries about itself, such as its author, creation date, device or location.

A MIME type (media type) names the format of content as type/subtype, such as text/html or image/png, so software knows how to handle it.

Minification removes whitespace, comments and other characters that are not needed to run code, making files smaller and faster to send.

Modulo gives the remainder after division: `a mod n` is what is left when a is divided by n.

Molarity is the concentration of a solution: moles of solute per litre of solution, written M.

Molecular weight, or molar mass, is the mass of one mole of a substance, found by adding the atomic weights of every atom in its formula.

The money factor is the finance charge used in a car lease; multiply it by 2,400 to get the approximate annual percentage rate.

A moon phase is how much of the Moon's sunlit face is visible from Earth, cycling from new moon to full moon and back about every 29.5 days.

Morse code encodes letters and digits as sequences of short and long signals, dots and dashes.

Mortgage points are fees paid upfront to lower a loan's interest rate; one point costs 1% of the loan amount.

Multi-factor authentication (MFA) requires two or more kinds of proof to sign in, such as a password plus a code from your phone or a security key.

A mutual fund pools money from many investors to buy a diversified portfolio of shares, bonds or other assets run by a manager.

An MX (mail exchanger) record is a DNS record that names the servers that accept email for a domain, with a priority number.

NAT (Network Address Translation) lets many devices with private IP addresses share one public IP address by rewriting addresses and ports as traffic passes through a router.

Net pay, or take-home pay, is what you receive after taxes and deductions are subtracted from your gross pay.

Net present value (NPV) is the sum of an investment's future cash flows, each discounted back to today's money, minus the initial cost.

Net worth is the total value of everything you own (assets) minus everything you owe (liabilities).

The normal distribution is the symmetric bell-shaped curve that describes many natural measurements, defined by its mean and standard deviation.

A number sequence is an ordered list of numbers that follows a rule; the common kinds are arithmetic (add a constant) and geometric (multiply by a constant).

OAuth is a standard that lets one app act on your behalf at another service, such as "Sign in with Google", without ever seeing your password.

OCR converts text in an image or scan into editable, searchable text.

Ohm's law states that the current through a conductor equals the voltage across it divided by its resistance: `V = I × R`.

A one-rep max is the heaviest weight you can lift for a single repetition; it is often estimated from a lighter set of several reps.

Open Graph is a set of meta tags (`og:title`, `og:description`, `og:image`) that control how a link looks when shared on social networks and messaging apps.

Overtime is time worked beyond the standard hours, usually paid at a higher rate such as time and a half (1.5 times the normal rate).

Ovulation is the release of an egg from an ovary, which happens about 14 days before the next period.

A p-value is the probability of seeing results at least as extreme as yours if there were really no effect; it is not the probability the result is true.

A pack-year is a measure of smoking exposure: the number of packs of 20 cigarettes smoked per day multiplied by the number of years smoked.

A packer compresses or encrypts an executable and adds a small stub that restores the original code in memory when the program runs.

A packet is a small unit of data sent across a network, made of a header with addressing information and a payload carrying the actual content.

A pay raise is an increase in pay, expressed as an amount or as a percentage of current pay.

PCAP is the file format for storing captured network packets, produced by tools such as tcpdump and Wireshark for later analysis.

PDF is a file format for documents that look the same on every device, bundling text, fonts, images and layout into one file.

Peak sun hours are the number of hours per day the sun delivers 1,000 watts per square metre, the unit used to size solar panels.

A pension is a retirement plan that pays a regular income for life, usually based on years of service and salary.

Percent error compares a measured value with the true value as a percentage: `|measured − true| / |true| × 100`.

A percentage expresses a number as a part of 100, so 25% means 25 out of every 100, or one quarter.

A permutation is an ordered arrangement of items; `P(n, k) = n! / (n − k)!` counts the ways to arrange k items chosen from n.

Phishing is a scam in which attackers pose as a trusted organisation or person, usually by email or text, to trick you into revealing credentials or money.

Pi is the ratio of a circle's circumference to its diameter, about 3.14159, the same for every circle.

Pixelation replaces blocks of pixels with a single average colour, producing a coarse mosaic effect.

A placeholder image is a stand-in picture of a set size and colour used while real images are not yet available.

PNG is a lossless image format with transparency, well suited to screenshots, logos and graphics with text or flat colours.

The Poisson distribution gives the probability of a number of events in a fixed interval when events occur independently at a constant average rate.

The Pomodoro Technique is a time-management method of working in focused 25-minute blocks separated by 5-minute breaks.

Pool volume is the amount of water a pool holds, found from its dimensions and average depth; one cubic foot holds about 7.48 US gallons.

A port is a number from 0 to 65535 that tells a computer which program or service an incoming network connection is for.

The Portable Executable format is the structure of Windows programs and libraries (.exe, .dll, .sys), with headers followed by named sections.

A prime number is a whole number greater than 1 whose only divisors are 1 and itself.

The principal is the original amount of money borrowed or invested, before any interest is added.

PRNU (photo response non-uniformity) is the tiny, fixed pattern of noise unique to a camera's image sensor, which can act as a fingerprint linking a photo to the camera that took it.

Probability is a number from 0 (impossible) to 1 (certain) that measures how likely an event is.

Profit margin is profit as a percentage of the selling price: `(price − cost) / price`.

Projectile motion is the curved path of an object thrown or launched and then moving only under gravity, ignoring air resistance.

A proxy is an intermediary server that passes requests and responses between a client and the destination.

A pseudo-random number comes from a deterministic algorithm that starts from a seed and produces a sequence that looks random and passes statistical tests.

Public-key (asymmetric) cryptography uses a pair of linked keys: a public key anyone can have, and a private key kept secret.

Punycode is the encoding that lets domain names with non-ASCII characters be stored in the ASCII-only DNS, marked by the prefix `xn--`.

In a right triangle the square of the hypotenuse equals the sum of the squares of the other two sides: `a² + b² = c²`.

A QR code is a two-dimensional barcode, a square grid of dark and light modules, that stores text or a link and is read by a phone camera.

The QT interval is the time on an ECG from the start of the heart's electrical contraction to the end of its recovery; QTc corrects it for heart rate.

The quadratic formula gives the solutions of `ax² + bx + c = 0`: `x = (−b ± √(b² − 4ac)) / 2a`.

A query string is the part of a URL after the question mark that carries named values for the server, such as ?q=shoes&page=2.

R-value measures a material's resistance to heat flow; a higher R-value means better insulation.

Rate limiting restricts how many requests a client can make to a service in a given time, to protect it from overload and abuse.

A ratio compares two quantities by division, written a:b, and is usually simplified by their greatest common factor.

A redirect is a server response that sends the browser to a different URL, using a 3xx status code and a Location header.

Refinancing is replacing an existing loan with a new one, usually to get a lower rate, a different term or to take out equity.

A regular expression (regex) is a pattern written in a compact syntax that describes a set of strings, used to search, validate and replace text.

Rent versus buy compares the total cost of renting a home with the total cost of owning one over the same period, including price growth.

The rent-to-income ratio is monthly rent divided by monthly income; a common guide is to keep it at or below 30%.

A required minimum distribution is the minimum yearly amount that must be withdrawn from most tax-deferred retirement accounts once the owner reaches a set age.

A resistor is a component that limits current in a circuit, with a value in ohms often shown by coloured bands.

Resolution is the number of pixels in an image (such as 1920 × 1080); pixel density (PPI) is how many pixels fit in an inch of a screen or print.

REST (representational state transfer) is a common style for web APIs in which resources have URLs and standard HTTP methods act on them.

Return on investment (ROI) is the gain or loss on an investment as a percentage of what was invested.

A reverse mortgage lets older homeowners borrow against their home equity, with no monthly payment; the loan is repaid when the home is sold or the owner leaves.

RGB describes colour as amounts of red, green and blue light, each from 0 to 255 on screens.

robots.txt is a public text file at a site's root that tells crawlers which paths they may or may not fetch.

A Roth IRA is a US retirement account funded with after-tax money, whose growth and qualified withdrawals are tax-free.

A router is a device that forwards data packets between different networks, choosing the path for each one.

The rule of 72 estimates how many years it takes money to double: divide 72 by the annual interest rate in percent.

Pace is the time taken to cover a distance unit, such as minutes per kilometre or per mile, the reciprocal of speed.

Sales tax is a percentage added to the price of goods at the point of sale, collected by the seller and passed to the government.

A salt is a unique random value added to a password before hashing, so that identical passwords produce different hashes.

The sample rate is how many times per second a digital recording measures the sound wave, in hertz.

Sample size is the number of observations in a study; bigger samples give narrower margins of error.

Scientific notation writes a number as a coefficient between 1 and 10 times a power of ten, which keeps very large and very small numbers readable.

Self-employment tax is the US Social Security and Medicare tax paid by people who work for themselves, covering both the employee and employer halves.

A SERP, or search engine results page, is the page a search engine shows in response to a query.

A shell is a program that reads commands you type, runs them and shows the results, forming the text interface to an operating system.

A short URL is a compact link that redirects to a longer address, made by a link-shortening service.

Significant figures are the digits in a number that carry meaning about its precision, starting from the first non-zero digit.

Simple interest is interest calculated only on the original amount, so it grows by the same sum each period.

An XML sitemap is a file listing a site's URLs, with optional last-modified dates, to help search engines find and crawl pages.

A sleep cycle is a roughly 90-minute pass through the stages of sleep; waking at the end of a cycle often feels easier than waking in the middle of one.

Slope is the steepness of a line: the change in y divided by the change in x between two points.

A slug is the readable, URL-safe part of an address that identifies a page, usually lowercase words joined by hyphens, such as how-dns-works.

Social engineering is manipulating people, rather than breaking technology, to get them to reveal information or take a harmful action.

Social Security is the US government retirement, disability and survivor benefit program, funded by payroll taxes.

A spectrogram is a picture of sound that plots time across, frequency up and the strength of each frequency as colour.

Speed is distance travelled divided by the time taken: `speed = distance / time`.

SPF (Sender Policy Framework) is a DNS record listing which servers may send email for a domain.

SQL injection is an attack in which input is crafted so that part of it is executed as database commands, letting an attacker read or change data.

SQLite is a lightweight database stored in a single file, used by browsers, phones and many apps to keep their data.

SSRF (server-side request forgery) is a vulnerability that lets an attacker make a server send requests to places it should not, such as internal systems.

Standard deviation measures how far values typically stray from their mean; it is the square root of the variance.

Steganography is the practice of hiding a message or file inside another ordinary-looking file, such as an image, so that nobody suspects it exists.

A subdomain is a name placed in front of a domain, such as blog.example.com or www.example.com, that can point to a different server or service.

A subnet mask is a 32-bit value that marks which part of an IPv4 address identifies the network and which part identifies the host.

SRT and WebVTT are plain-text subtitle formats that pair lines of dialogue with start and end times.

Sunrise and sunset are the times the top edge of the Sun crosses the horizon, which depend on date and latitude.

SVG (scalable vector graphics) is an XML-based image format that describes shapes rather than pixels, so it stays sharp at any size.

A t-test judges whether the difference between means, from one or two small samples, is bigger than chance would explain.

TCP (Transmission Control Protocol) is the transport protocol that delivers data reliably and in order between two programs.

TDEE is total daily energy expenditure: the calories you burn in a day including movement, found by multiplying BMR by an activity factor.

A diff compares two texts and lists what was added, removed or left unchanged, usually line by line.

A time card records the hours an employee works each day, from which total hours and pay are calculated.

Timestomping is deliberately changing a file's timestamps to hide when it was created or modified, a technique attackers use to blend in with older files.

A tire size such as 225/45R17 gives the width in millimetres (225), the sidewall height as a percentage of width (45) and the rim diameter in inches (17).

TLS (Transport Layer Security) is the cryptographic protocol that encrypts and authenticates connections, including HTTPS, email and VPNs.

Trigonometry relates the angles of a triangle to the ratios of its sides through sine, cosine and tangent.

TSV is a plain-text table format in which each line is a row and the columns are separated by tab characters.

TTL (time to live) is how long, in seconds, a DNS record or cached item may be reused before it must be fetched again.

UDP (User Datagram Protocol) is a fast, connectionless transport protocol that sends data without guaranteeing delivery or order.

Unicode is the universal standard that gives every character in the world's writing systems, plus symbols and emoji, a unique number called a code point.

A unique local address is a private IPv6 address in `fc00::/7`, used inside a site or between sites but not routed on the public internet.

Unit price is the cost per standard unit of measure, such as per litre or per 100 g, which lets you compare differently sized packages fairly.

A Unix timestamp is the number of seconds since 00:00:00 UTC on 1 January 1970, the "Unix epoch", used by computers as a simple way to store a moment in time.

A URL (Uniform Resource Locator) is the address of a resource on the web, such as a page, image or API endpoint.

A user agent is the software making a web request, such as a browser or crawler, which identifies itself to servers in the User-Agent header.

UTC is the global reference time standard from which all time zones are defined as offsets.

UTF-8 is the dominant Unicode encoding, writing each character as one to four bytes and staying identical to ASCII for the first 128 characters.

A UUID (universally unique identifier) is a 128-bit value, written as 36 characters such as 123e4567-e89b-12d3-a456-426614174000, that is unique for practical purposes without a central registry.

A VA loan is a US mortgage guaranteed by the Department of Veterans Affairs for eligible service members, veterans and some spouses, often with no down payment.

Variance is the average of the squared distances of values from their mean, a measure of how spread out they are.

VAT is a consumption tax added to the price of goods and services at each stage of production, ultimately paid by the final consumer.

A vCard (.vcf file) is a standard text format for contact details that phones and email programs can import.

A vector is a quantity with both a size and a direction, written as a list of components such as (3, 4).

VO₂ max is the maximum rate at which the body can use oxygen during hard exercise, a standard measure of aerobic fitness in mL per kg per minute.

A voltage divider is two resistors in series that produce a lower output voltage: `Vout = Vin × R2 / (R1 + R2)`.

A VPN (virtual private network) creates an encrypted tunnel between your device and another network, so traffic crosses the internet privately.

A vulnerability is a weakness in software, hardware or a process that an attacker could exploit to do something unintended.

The waist-to-hip ratio divides waist circumference by hip circumference, a measure of where body fat is carried.

A watermark is text or a logo laid over an image or document to show ownership or discourage reuse.

WebP is a modern image format from Google that supports lossy and lossless compression, transparency and animation in smaller files than JPEG or PNG.

WebSocket is a protocol that keeps a single connection open between a browser and a server so both can send messages at any time.

A weighted average gives each value a weight so that more important values count for more: `Σ(weight × value) / Σweight`.

The Wilks score is a coefficient-based number used in powerlifting to compare lifters of different body weights and sexes.

Wind chill is the "feels like" temperature that shows how cold exposed skin feels when wind carries heat away.

XML (Extensible Markup Language) is a text format that describes data with nested, named tags, such as <name>Ana</name>.

YAML is a human-friendly data format that uses indentation instead of brackets, popular for configuration files.

A z-score says how many standard deviations a value lies above or below the mean: `z = (x − mean) / SD`.

A zero-day is a vulnerability unknown to the software's maker, or without an available fix, that attackers can exploit before anyone can defend against it.

A 401(k) is a US employer-sponsored retirement account to which you contribute from your paycheck, often with an employer match, and which grows tax-deferred.