From IHT:

There’s a name for this: user interface design, or UI for short. Its goal is to ensure that all of us can use digital devices simply and intuitively, regardless of how techno- savvy we are. As we’re spending more and more time with digital products, UI is becoming one of the most important areas of design, but it’s also one of the trickiest to judge.

http://www.iht.com/articles/2007/02/25/features/DESIGN26.php?page=1