The client application used with a particular network protocol, such as http: or ftp:.
Definition #2 for "user agent"
In computing, a user agent is software that is acting on behalf of a user. For example, an email reader is a Mail User Agent, and in the Session Initiation Protocol, the term user agent refers to both end points of a communications session. In many cases, a user agent acts as a client in a network protocol used in communications within a client–server distributed computing system. In particular, the Hypertext Transfer Protocol identifies the client software originating the request, using a "User-Agent" header, even when the client is not operated by a user. The SIP protocol followed this usage.