An interface is an element or trait which links two or more elements by creating a specific functional relationship between them.
The term interface is generally useful in modeling systems, although it's often especially associated with software development. Interfaces turn potential relationships into active relationships in specific conditions, creating systems. The elements which are linked by interfaces become components in the activated system.
Designed interfaces, like those in the aforementioned software development, often enable agents to control (and use) complex tools.