Interface
A designed mediator that enables interaction and communication between a system and its user.
In practice
An interface is anything designed to sit between a person and a system so they can communicate: an app's screens and buttons, an oven's knobs and display, a car's dashboard. Its job is to translate between what the system can do and what a person understands. A good stove interface makes it obvious which knob controls which burner, without labels that need reading.
Often confused with User Interface (UI)
Interface is the general idea of a mediator between system and user; UI refers specifically to the interactive elements of a digital system.
Try this
Sketch the controls of a kitchen appliance and mark the ones whose purpose you had to learn by trial and error.