Service Interface

Here's the definition I tend to use:

A service interface describes how to invoke the functionality of a service. It can be invoked by a service requestor of the service and must be implemented by a service provider of a service. The interface enumerates the service's operations, including the data format of each operation's parameters and return type. A service's interface can be explicitly declared, such as a WSDL port type, or implied, such as a REST interface with standard HTTP operations and a description of their data formats.