There is still some discussion over the definitions of the four terms: Integration, Interoperability, Compatibility and Portability.  The aim of this note is to provide an explanation of the four terms as used by the Testing Standards Working Party.
To explain the terms then two basic entities are required:  Components are one of the parts that make up a system, while a system is a collection of components organised to accomplish a specific function or a set of functions (both from IEEE 610).
Integration is concerned with the process of combining components into an overall system (after IEEE 610).  In software, we are normally concerned with integration at two levels.  First there is the integration of components at the module level into a system – sometimes known as component integration testing or integration in the small.  Second there is the integration of systems into a larger system – sometimes known as system integration testing or integration testing in the large.
Integration testing is all encompassing in that it is concerned not only with whether the interface between components is correctly implemented, but also with whether the integrated components – now a system - behave as specified.  This behavior will cover both functional and non-functional aspects of the integrated system.