A response header is an HTTP header that can be used in an HTTP response and that doesn't relate to the content of the message. Response headers, like Age, Location or Server are used to give a more detailed context of the response.
Not all headers appearing in a response are response headers. For example, the Content-Length header is an entity header referring to the size of the body of the response message. However, these entity requests are usually called responses headers in such a context.
The following shows a few response headers after a GET request. Note that strictly speaking, the Content-Encoding and Content-Type headers are entity header:
200 OK Access-Control-Allow-Origin: * Connection: Keep-Alive Content-Encoding: gzip Content-Type: text/html; charset=utf-8 Date: Mon, 18 Jul 2016 16:06:00 GMT Etag: "c561c68d0ba92bbeb8b0f612a9199f722e3a621a" Keep-Alive: timeout=5, max=997 Last-Modified: Mon, 18 Jul 2016 02:36:04 GMT Server: Apache Set-Cookie: mykey=myvalue; expires=Mon, 17-Jul-2017 16:06:00 GMT; Max-Age=31449600; Path=/; secure Transfer-Encoding: chunked Vary: Cookie, Accept-Encoding X-Backend-Server: developer2.webapp.scl3.mozilla.com X-Cache-Info: not cacheable; meta data too large X-kuma-revision: 1085259 x-frame-options: DENY