What is a user interface?

A lot of people I talk to are confused about design. Not least when they hear about abstract design such as web design, UX design, game design etc. I can't blame them. As designers we really tag ourselves with the word most appropriate for the task at hand. Even though our main work is always to solve problems by design. But let's make things easier For most designers working with abstract design the term user interface is crucial. But exactly what is a UI? Sure, it's the thing the user interacts with. But where does it start and where does it end?

User Interface Interface is a proxy layer between a human being and a function.

But what does that mean? For a pair of scissors, the scissors themselves are the user interface between a human hand and the function of cutting.

A computer has two layers of user interfaces between the human and most functions. The keyboard/mouse or physical UI, and the graphical or text based abstract UI.

But what if the user interface is a part of the function? The iPhone for instance doesn't really have a physical UI. There is nothing physical to interact with (excepting the home button, volume and mute controls but lets not digress from the example). But it does have a graphical abstract UI.

Why is this definition important? Because now we can all say user interface and know what we're referring to. No more wordplay to guess what the other person is talking about.