• An HTTP header that can be used in an HTTP request, and that does not relate to the content of the message. Request headers, like Accept, Accept-*, or If-*, allow to perform conditional requests; others like Cookie, User-Agent, or Referer narrow down the context so that the server can tailor the answer. Not all headers appearing in a request are request headers. For example, the Content-Length appearing in a POST request is actually an entity header referring to the size of the body of the request message. However, these entity headers are often called request headers in such a context. ← MDN Web Docs

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