---
title: "Request header - Glossary | MDN"
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source: "https://developer.mozilla.org/en-US/docs/Glossary/Request_header"
category: "reference"
tags: "web, html, css, javascript, documentation"
date_saved: "2026-05-05T05:42:53.981590+00:00"
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  1. [Glossary](https://developer.mozilla.org/en-US/docs/Glossary)
  2. [Request header](https://developer.mozilla.org/en-US/docs/Glossary/Request_header)

# Request header
A **request header** is an [HTTP header](https://developer.mozilla.org/en-US/docs/Glossary/HTTP_header) that can be used in an HTTP request to provide information about the request context, so that the server can tailor the response. For example, the [`Accept-*`](https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/Accept) headers indicate the allowed and preferred formats of the response. Other headers can be used to supply authentication credentials (e.g., [`Authorization`](https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/Authorization)), to control caching, or to get information about the user agent or referrer, etc.
Not all headers that can appear in a request are referred to as _request headers_ by the specification. For example, the [`Content-Type`](https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/Content-Type) header is referred to as a [representation header](https://developer.mozilla.org/en-US/docs/Glossary/Representation_header).
In addition, [CORS](https://developer.mozilla.org/en-US/docs/Glossary/CORS) defines a subset of request headers as [simple headers](https://developer.mozilla.org/en-US/docs/Glossary/CORS-safelisted_request_header), request headers that are always considered authorized and are not explicitly listed in responses to [preflight](https://developer.mozilla.org/en-US/docs/Glossary/Preflight_request) requests.
The HTTP message below shows a few request headers after a [`GET`](https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Methods/GET) request:
## In this article
  * [See also](https://developer.mozilla.org/en-US/docs/Glossary/Request_header#see_also)

##  [See also](https://developer.mozilla.org/en-US/docs/Glossary/Request_header#see_also)
  * [List of all HTTP headers](https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers)
  * [RFC 9110, section 6.3: Header Fields](https://httpwg.org/specs/rfc9110.html#header.fields)