An interface defines the interaction between certain modules.
Interface is a very general concept which refers to the interaction points of arbitrary modules:
The interface defines how a module shall be used. There may be ways circumventing the interface and accessing internal parts of a module directly. This should be avoided (IH/E but is sometimes done.
Note that in this wiki whenever the language construct is meant (and not the concept)
interface shall be written using a monospace font:
interface vs. interface.