Glossary
78 terms from "HTTP basics: reading requests and responses in the browser." Look them up when you get stuck; the first mention in the text carries a hover definition.
| Term | Definition | Source |
|---|---|---|
| HTTP | The Hypertext Transfer Protocol, an application-layer protocol for fetching resources such as HTML documents between a client and a server. | MDN: Overview of HTTP |
| client-server protocol | A protocol in which one side always initiates the exchange and the other only answers; in HTTP the browser initiates and the server answers. | MDN: Overview of HTTP |
| browser (client) | The user-agent program that sends HTTP requests to a server and renders the returned response into a web page. | MDN: Overview of HTTP |
| server | A program or machine that listens for HTTP requests and sends back responses, often after processing the requested resource. | MDN: Overview of HTTP |
| URL | Uniform Resource Locator; a web address that identifies a server and the resource to request from it. | MDN: What is a URL? |
| scheme | The first part of a URL, before the `://`, naming the protocol the browser must use — usually HTTPS or HTTP. | MDN: What is a URL? |
| HTTPS | HTTP carried over TLS, so that the communication between client and server is encrypted. | MDN: HTTPS (glossary) |
| authority | The part of a URL after the `://`, containing the domain and optionally a port, naming which server to reach. | MDN: What is a URL? |
| port | The numbered gate on a server used to reach its resources; omitted from a URL when it is the protocol's standard port (80 for HTTP, 443 for HTTPS). | MDN: What is a URL? |
| query string | The part of a URL after the `?`, carrying key/value parameters the server can use before returning the resource. | MDN: What is a URL? |
| fragment | The part of a URL after the `#`; a bookmark inside the resource that is never sent to the server. | MDN: What is a URL? |
| resource | Whatever a URL names and a request asks for: an HTML page, an image, a stylesheet, or a data payload. | RFC 9110: HTTP Semantics |
| origin server | The server that owns the requested resource and is authoritative for answering about it. | RFC 9110: HTTP Semantics |
| round trip | One complete request-and-response pair; the unit of delay that page loads and redirects are counted in. | MDN: Redirections in HTTP |
| HTTP request | A message sent by the client to the server asking it to perform an action on a resource. | RFC 9110: HTTP Semantics |
| HTTP response | A message sent by the server back to the client after receiving a request, including a status code and often a body. | RFC 9110: HTTP Semantics |
| HTTP method | A word in the request line that indicates the action the client wants the server to perform, such as GET or POST. | MDN: HTTP request methods |
| request line | The first line of a request, made of three parts: the method, the request target, and the protocol version. | MDN: HTTP messages |
| request target | The second slot of the request line: the path plus query string naming what is being asked for. | MDN: HTTP messages |
| GET | The HTTP method that requests a representation of a resource; it should only retrieve data and not contain a body. | MDN: GET request method |
| POST | The HTTP method that submits data to a server, often causing a change in state or side effects. | MDN: POST request method |
| PUT | The method that replaces the current representation of the target resource with the request content. | MDN: HTTP request methods |
| DELETE | The method that deletes the specified resource. | MDN: HTTP request methods |
| HEAD | The method that asks for a response identical to GET but without the response body. | MDN: HTTP request methods |
| safe | A property of a method that is read-only: it is not supposed to change anything on the server. GET and HEAD are safe; POST is not. | MDN: HTTP request methods |
| idempotent | A property of a method whose repetition leaves the same end result as performing it once. GET, PUT, and DELETE are idempotent; POST is not. | MDN: HTTP request methods |
| cacheable | A property of a method whose response may be stored and reused; GET is cacheable by default, POST only conditionally. | MDN: HTTP request methods |
| header | A line of metadata in an HTTP message that gives the recipient extra information about the request or response. | MDN: HTTP messages |
| Host | The request header naming the domain the request is for, since the request line carries only the path. | MDN: HTTP messages |
| User-Agent | The request header identifying which browser is asking. | MDN: HTTP messages |
| Content-Type | The header that states the format of the message body so the recipient knows how to parse it. | MDN: HTTP messages |
| Content-Length | The header stating how many bytes the message body contains. | MDN: HTTP messages |
| Set-Cookie | The response header that asks the browser to store a value and send it back on later requests. | MDN: Using HTTP cookies |
| Cookie header | The request header on which the browser returns stored cookie values to the server. | MDN: Using HTTP cookies |
| Domain | The cookie attribute specifying which server may receive the cookie; if set, the cookie also goes to that domain's subdomains. | MDN: Using HTTP cookies |
| status line | The first line of a response, carrying the protocol version, the status code, and an optional reason phrase. | MDN: HTTP messages |
| status code | A three-digit number in a response that indicates whether the request succeeded or failed and why. | MDN: HTTP response status codes |
| reason phrase | The short human-readable text after the status code; purely informational, since the number carries the meaning. | MDN: HTTP messages |
| 2xx | The success class of status codes: the request arrived, was understood, and was answered. | MDN: HTTP response status codes |
| 4xx | The client error class: the server was reached and understood the request but something about the request was wrong. | MDN: HTTP response status codes |
| 5xx | The server error class: the server itself failed while trying to fulfill an otherwise valid request. | MDN: HTTP response status codes |
| 200 OK | A successful response status code indicating that the request succeeded and the response body contains the requested resource. | MDN: 200 OK |
| Location | The response header on a redirect holding the URL the browser should load next. | MDN: Redirections in HTTP |
| Cache-Control | The header stating whether and for how long a response may be stored and reused. | MDN: HTTP caching |
| private cache | A cache tied to one client, typically the browser's own, which may therefore store a personalized response. | MDN: HTTP caching |
| shared cache | A cache sitting between clients and the server that can store responses reusable by many users. | MDN: HTTP caching |
| no-store | The Cache-Control directive that forbids keeping any copy of the response. | MDN: HTTP caching |
| no-cache | The Cache-Control directive that allows storing a response but requires validating it with the server before reuse. | MDN: HTTP caching |
| revalidation | The check a client makes with the server to confirm a stored response is still usable before reusing it. | MDN: HTTP caching |
| 304 Not Modified | The response to a successful revalidation: no body, meaning the copy the client already holds is still good. | MDN: Redirections in HTTP |
| 204 No Content | A success status code whose response has no body, because the status and headers already say everything. | MDN: HTTP messages |
| developer tools (DevTools) | A set of tools built into browsers for inspecting pages, including the Network panel that logs HTTP requests. | Chrome DevTools: Inspect network activity |
| Network panel | The panel in browser DevTools that records and displays every HTTP request made while the tools are open. | Chrome DevTools: Inspect network activity |
| Initiator | The Network panel column showing what caused a request: the HTML parser, a script, or a redirect. | Chrome DevTools: Network panel overview |
| disk cache | The value shown in the Size column when a response was served from the browser's own stored copy rather than the network. | MDN: HTTP caching |
| Preserve log | The Network panel option that keeps recorded requests across navigations instead of clearing them. | Chrome DevTools: Inspect network activity |
| Headers tab | The detail tab for a selected request, showing the request line, request headers, and response headers as actually sent. | Chrome DevTools: Inspect network activity |
| document request | The first request of a page load, whose response body is the HTML document itself; its Type is `document`. | MDN: Overview of HTTP |
| subresource | A resource such as an image, stylesheet, or script that the browser requests after parsing the main HTML document. | MDN: Overview of HTTP |
| waterfall | The timing visualization in the Network panel that shows when each request started and how long it took. | Chrome DevTools: Network panel overview |
| critical rendering path | The sequence of steps by which the browser converts HTML, CSS, and JavaScript into pixels on the screen. | MDN: Critical rendering path |
| first paint | The moment the browser puts the page's initial pixels on screen; some subresources must arrive before it can happen. | MDN: Critical rendering path |
| dependency | A relationship in which one request cannot start until an earlier response has been received and parsed. | Chrome DevTools: Network panel overview |
| 404 Not Found | A client error status code indicating that the server cannot find the resource requested by the URL. | MDN: 404 Not Found |
| 403 Forbidden | A client error status code meaning the server understood the request but refuses to process it, typically for lack of permission. | MDN: 403 Forbidden |
| 401 Unauthorized | A client error status code where authenticating is the remedy, unlike 403 where it changes nothing. | MDN: 403 Forbidden |
| 500 Internal Server Error | A server error status code indicating that the server encountered an unexpected condition that prevented it from fulfilling the request. | MDN: 500 Internal Server Error |
| 301 Moved Permanently | A permanent redirect: the original URL should no longer be used, and crawlers update their stored URL. | MDN: Redirections in HTTP |
| 302 Found | A temporary redirect: crawlers keep the original URL, and non-GET methods may or may not be rewritten to GET. | MDN: Redirections in HTTP |
| 307 Temporary Redirect | A temporary redirect that guarantees the method and body are not changed. | MDN: Redirections in HTTP |
| 308 Permanent Redirect | A permanent redirect that guarantees the method and body are not changed. | MDN: Redirections in HTTP |
| 303 See Other | A redirect that deliberately changes other methods to GET, so refreshing the result page does not re-trigger the operation. | MDN: Redirections in HTTP |
| CORS | Cross-Origin Resource Sharing; a header-based mechanism by which a server tells the browser which other origins may load its responses. | MDN: Cross-Origin Resource Sharing (CORS) |
| origin | The combination of domain, scheme, and port that identifies where content came from, and the unit CORS decisions are made in. | MDN: Cross-Origin Resource Sharing (CORS) |
| same-origin policy | The browser rule restricting cross-origin HTTP requests initiated from scripts; CORS is the mechanism for relaxing it. | MDN: Cross-Origin Resource Sharing (CORS) |
| Access-Control-Allow-Origin | The response header by which a server states which origin is permitted to read the response. | MDN: Cross-Origin Resource Sharing (CORS) |
| preflight | An OPTIONS request the browser sends first, asking whether the real cross-origin request will be permitted. | MDN: Cross-Origin Resource Sharing (CORS) |
| TLS | The layer HTTPS uses to encrypt communication between client and server. | MDN: HTTPS (glossary) |