Glossary — B
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Backend
The part of an application that runs on a server (not the user’s browser). Handles data storage, business logic, authentication, anything that needs to be trusted or kept secret. The browser-facing half is called the frontend.
See also: Backend (section)
Bash
The classic Unix shell — the program that takes typed commands and runs them. On macOS and Linux, Bash (or its newer cousin Zsh) is the default. On Windows, the equivalent is PowerShell, though Bash is available via Git Bash or WSL. Most tutorials use Bash syntax, which often needs translation on Windows.
See also: PowerShell vs Bash, Terminal
Big O (notation)
A way to describe how an algorithm’s running time grows as the input gets bigger. O(1) is constant time (always takes the same amount of time). O(n) is linear (twice as much data → twice as long). O(n²) is quadratic (twice as much data → four times as long — bad for big inputs). You don’t need to memorize it, but you do need to recognize when something is “going to get really slow really fast.”
See also: Time & space complexity
Binary
Base-2 numbers — only 0s and 1s. The native language of computers, because each digit corresponds to “off” or “on” in an electrical circuit. The number 10 in binary is the same as 1010. You rarely write binary directly; it’s mostly relevant when reading file sizes (1 KB = 1024 bytes) or bitwise operations.
See also: What is a computer?
Boolean
A data type with exactly two possible values: true or false. Named after George Boole, the 19th-century mathematician who formalized true/false logic. Used for any “is this on or off” check: isLoggedIn, darkMode, hasPermission.
See also: JavaScript
Branch (Git)
A parallel line of development in a Git repository. The default branch is usually called main (formerly master). You create a new branch to work on a feature without touching the main code, then merge it back in when it’s ready. Branches make experimentation cheap — if it goes badly, you delete the branch.
See also: Branches & merging, Git basics
Browser
The program a user uses to view web pages: Chrome, Firefox, Safari, Edge, Arc, Brave. All modern browsers are built on a small number of underlying engines — Chromium (Chrome, Edge, Arc, Brave), Gecko (Firefox), WebKit (Safari). They render HTML, run JavaScript, store data, and make network requests on behalf of the page.
See also: How the web works
Bug
A defect in software — a mistake that causes it to behave wrongly. The term famously comes from a real moth that Grace Hopper’s team taped to a logbook in 1947 after extracting it from a relay in the Harvard Mark II computer. Today’s bugs are almost always conceptual, not actual insects.
See also: Debug (D), Testing & quality
Build
The process of turning your source code into the optimized files that actually get served to users. For a Next.js project, npm run build runs the build: TypeScript is type-checked, JSX is compiled to JavaScript, files are bundled, CSS is processed, and the result lands in a .next/ folder ready for the server.
See also: Deploy a Next.js app to Vercel
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