What
"Integration testing where the interfaces between system components are tested."
BS 7925-1.British Computer Society Specialist Interest Group in Software Testing (BCS SIGIST)

Integration of "components" can take place at many stages of the lifecycle. The process can be "in the small" or "in the large". The size refers to component. In the small means integration of the smallest components, to make sub-assemblies. At the other extreme, in the large, is the integration of entire systems. BS 7925-1 however seems only to concentrate on the smaller of the two.

Interface testing is primarily intersted in the interaction between two or more components.