a named set of abstract operations and messages involved

a named set of abstract operations, defined using abstract message types

An interface which is a logical grouping of operations and represents an abstract service type, independent of transport protocol and data format.

A property that defines the set of message interaction patterns called operations that are permitted at that end point. An operation can be one-way, in which case one message is sent or received, or it could be request-response in which case a message is sent (or received) and a response is received (or sent).