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Glossary — C

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Cache

A temporary store of recently-accessed data, kept somewhere faster than the original source. Browsers cache images and CSS so the next visit is fast. CDNs cache pages near users. Databases cache common query results. The famous joke: “There are only two hard things in computer science: cache invalidation and naming things.”

See also: CDN, Hosting & deployment

Callback

A function that you pass to another function so it can call you back later, usually when something finishes. The old way of doing async work in JavaScript, before Promises and async/await. Famous for producing “callback hell” — nested callbacks 5 levels deep.

fs.readFile("a.txt", (err, data) => {
  if (err) return console.error(err);
  console.log(data);
});

See also: Async, Promise

CDN (Content Delivery Network)

A network of servers spread around the world, each holding cached copies of your site’s assets (images, CSS, JS bundles, sometimes HTML). When a user requests something, they get it from the nearest CDN server, not from your origin server. This makes the site fast everywhere — not just near wherever you host. Vercel and Cloudflare are CDNs (among other things).

See also: CDNs, Vercel

CI/CD (Continuous Integration / Continuous Deployment)

The automated pipeline that runs every time you push code: tests run, builds happen, and if everything passes, the new version is deployed. CI is the testing/build half; CD is the deploy half. Vercel does both automatically for Next.js projects. The phrase is one of the most-uttered acronyms in modern dev work.

See also: CD

Claude

Anthropic’s family of large language models. As of 2026, the lineup includes Opus (most capable), Sonnet (the daily-driver balance), Haiku (fast and cheap), and Fable (specialized). Claude is what powers the CLI agent (Claude Code) and the chat interface at claude.ai.

See also: Claude models, Anthropic

Claude Code

Anthropic’s official CLI tool. It runs in your terminal, reads your project, edits files, runs commands, and remembers context across sessions via memory files. The whole encyclopedia you are reading was built with it.

See also: Claude Code deep dive, Memory system, Slash commands

Client

In the “client/server” model: the side that asks for things. A browser is a client. A mobile app is a client. Even a Postgres command-line tool is a client (with Postgres being the server). Compare with server.

See also: Client vs server

Cloud

A loose term for “computers we rent from someone else, usually accessed over the internet.” When people say “deployed to the cloud” they usually mean AWS, Google Cloud, Azure, Vercel, or similar — as opposed to a server you own and physically own.

See also: What is “the cloud”?, Vercel

Component (React)

A reusable piece of UI in React. A function (or class) that returns JSX. Components compose — you build a <Page> out of <Header>, <Sidebar>, <Content>, <Footer>, each of which might be made of smaller components.

function Greeting({ name }) {
  return <h1>Hello, {name}</h1>;
}

See also: React, JSX

A small piece of data the browser stores on behalf of a website, and sends back to that website on every subsequent request. The classic way to keep someone “logged in.” Modern cookies have flags like HttpOnly (JS can’t read it), Secure (only sent over HTTPS), and SameSite (controls cross-site sending) that are critical for security.

See also: Sessions & cookies

CORS (Cross-Origin Resource Sharing)

The browser security policy that controls when JavaScript on one domain is allowed to talk to a different domain. By default, browsers refuse — the server has to send specific headers (like Access-Control-Allow-Origin) saying “yes, this other site is allowed to call me.” Cause of approximately 50% of all “but it works in Postman!” confusion.

See also: HTTP, APIs overview

CSS (Cascading Style Sheets)

The language that controls how HTML looks: colors, fonts, sizes, spacing, layout, animations. HTML provides the structure; CSS provides the styling. Modern CSS includes Flexbox and Grid for layout, custom properties (variables), and is plenty powerful on its own — though most modern projects use Tailwind to write CSS faster.

See also: CSS (textbook), Tailwind

CSV (Comma-Separated Values)

A plain-text file format where each row is a record and each value is separated by commas. The lowest-common-denominator way to exchange tabular data. Excel and Google Sheets both open it natively.

name,age,city
George,99,Sydney

See also: JSON

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