What is an interface? Is it something that a user sees on a screen? Is it the border between different elements? Is it a way to get to some other place?
An interface is the common boundary of two entities. It’s the place where independent entities communicate with each other.
Interfaces are everywhere. When you walk and feel the floor beneath your feet there’s an interface between your body and the floor. When you talk to someone, your voice is the interface between yourself and the other person.
Sometimes our perception of what an interface is is wrong. The way we perceive entities deceives us from what they are in reality. Take breathing, for instance. When you breathe, your lungs are the interface between your blood and the air surrounding you. The interface is not your body, as the air also flows inside.