An interface is the point of communication between user and system, allowing for input to the system and feedback to the user.
An interface facilitates comminucation between a user and a system, like a machine or software.
An interface can be physical (like a steering wheel, gearstick and speed meter in a car) or virtual (like a computer OS).
In screen design, the term interface often refers to the more specific GUI (Graphical User Interface). Questions dealing with the design of such GUIs can be tagged with the more specific gui-design.
Questions with the interface tag are concerned with more general aspects of interfaces and potentially include questions about physical or hybrid interfaces.