Except for *mirror* all definitions are taken directly from the
HTTP 1.1 and HTTP Duplicate Suppression Internet-Drafts.

gateway
    A server which acts as an intermediary for some other server.
    Unlike a proxy, a gateway receives requests as if it were the
    origin server for the requested resource; the requesting client
    may not be aware that it is communicating with a gateway.

origin server
    The server on which a given resource resides or is to be created.

resource
    A network data object or service that can be identified by a URI.

instance
    The entity that would be returned in a status-200 response to a
    GET request, at the current time, for the selected variant of the
    specified resource, but without the application of any
    content-coding or transfer-coding.

mirror
    Informally, one of multiple origin servers or URIs providing the
    same instance body.