Overview#Proxy is defined by for our use as an Entity authorized to act on behalf of another. (Which is Delegation) Proxy Servers are by definition an Observer and a potential Man-In-The-Middle RFC 2616 defines Proxy as an intermediary program which acts as both a server and a client for the purpose of making requests on behalf of other clients.
Requests are serviced internally or by passing them on, with possible translation, to other servers. A proxy MUST implement both the client and server requirements of this specification.
A "non-transparent proxy" is a Proxy that modifies the request or response in order to provide some added service to the user-agent, such as group annotation services, media type transformation, protocol reduction, or anonymity filtering. Except where either transparent or non-transparent behavior is explicitly stated, the HTTP Proxy requirements apply to both types of proxies.