Without looking anything up:

"Depends in which context we are talking about it, but in the broadest sense it is a defined systemic boundary between two different domains. E.g. a User interface marks the boundary between the user and the program, an API could mark the boundary between the program and other programs, an ABI can mark the boundary between binary data and programs who read that data and so on. Ideally you want those interfaces to be well defined as theh act as translation layers and other systems can start to rely on the shape of an interface. "

I could then talk about why interfaces can act as a decoupling layer etc.