Request header

A request header is an 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-* headers indicate the allowed and preferred formats of the response. Other headers can be used to supply authentication credentials (e.g., 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 header is referred to as a representation header.