Interface
The external touch point between applications to collaborate or share functional behavior. Interfaces are completely described by the combination of logical and physical portions of a WSDL document.
Once defined in a contract, an interface is the abstract boundary that a service exposes. A service's interface is the set of message types and message exchange patterns through which service consumers can interact with that service. In a WSDL 1.0 document, interfaces are defined using the portType element.