Most of the time, when I tell people I'm a UI designer they ask "what's that?" Often, what follows is some bang-up job, but I really like Wikipedia's take on User Interface Design.
Oh, don't forget Interaction Design, Information Design, Information Architecture, User Experience Design, Human Factors, Human-Computer interaction & Information Visualization. OK, I know these all have subtle differences but still. Can we stop the madness?
Unlike computer science which tends to overload terms, to assign many different meanings to one word, User experience suffers from term dispersement. Too many terms for the same or very similar things.