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+---
+title: HTTP header
+slug: Glossary/HTTP_header
+page-type: glossary-definition
+---
+
+{{GlossarySidebar}}
+
+An **HTTP header** is a field of an HTTP request or response that passes additional context and metadata about the request or response. For example, a request message can use headers to indicate it's preferred media formats, while a response can use header to indicate the media format of the returned body. Headers are case-insensitive, begin at the start of a line and are immediately followed by a `':'` and a header-dependent value. The value finishes at the next CRLF or at the end of the message.
+
+The HTTP and Fetch specifications refer to a number of header categories, including:
+
+- {{Glossary("Request header")}}: Headers containing more information about the resource to be fetched or about the client itself.
+- {{Glossary("Response header")}}: Headers with additional information about the response, like its location or about the server itself (name, version, …).
+- {{Glossary("Representation header")}}: metadata about the resource in the message body (e.g. encoding, media type, etc.).
+- {{Glossary("Fetch metadata request header")}}: Headers with metadata about the resource in the message body (e.g. encoding, media type, etc.).
+
+A basic request with one header:
+
+```http
+GET /example.html HTTP/1.1
+Host: example.com
+```
+
+Redirects have mandatory headers ({{HTTPHeader("Location")}}):
+
+```http
+302 Found
+Location: /NewPage.html
+```
+
+A typical set of headers:
+
+```http
+304 Not Modified
+Access-Control-Allow-Origin: *
+Age: 2318192
+Cache-Control: public, max-age=315360000
+Connection: keep-alive
+Date: Mon, 18 Jul 2016 16:06:00 GMT
+Server: Apache
+Vary: Accept-Encoding
+Via: 1.1 3dc30c7222755f86e824b93feb8b5b8c.cloudfront.net (CloudFront)
+X-Amz-Cf-Id: TOl0FEm6uI4fgLdrKJx0Vao5hpkKGZULYN2TWD2gAWLtr7vlNjTvZw==
+X-Backend-Server: developer6.webapp.scl3.mozilla.com
+X-Cache: Hit from cloudfront
+X-Cache-Info: cached
+```
+
+> **Note:** Older versions of the specification referred to:
+>
+> - {{Glossary("General header")}}: Headers applying to both requests and responses but with no relation to the data eventually transmitted in the body.
+> - {{Glossary("Entity header")}}: Headers containing more information about the body of the entity, like its content length or its MIME-type (this is a superset of what are now referred to as the Representation metadata headers)
+
+## See also
+
+- [List of all HTTP headers](/en-US/docs/Web/HTTP/Headers)
+- Syntax of [headers](https://datatracker.ietf.org/doc/html/rfc7230#section-3.2) in the HTTP specification
+- [Glossary](/en-US/docs/Glossary)
+
+  - {{Glossary("HTTP header")}}
+  - {{Glossary("Request header")}}
+  - {{Glossary("Response header")}}
+  - {{Glossary("Representation header")}}
+  - {{Glossary("Fetch metadata request header")}}
+  - {{Glossary("Forbidden header name")}}
+  - {{Glossary("Forbidden response header name")}}
+  - {{Glossary("CORS-safelisted request header")}}
+  - {{Glossary("CORS-safelisted response header")}}