An Interface

1. The point of interconnection or contact between entities.
2. (chemistry, physics) A thin layer or boundary between different substances or two phases of a single substance.
3. (computing) The point of interconnection between systems or subsystems.
4. (computing) The connection between a user and a machine.
5. (computing, object-oriented programming) The connection between parts of software.
6. (computing, object-oriented programming) In object-oriented programming, a piece of code defining a set of operations that other code must implement.
7. (biochemistry) The internal surface of a coiled protein (compare exoface).