stands for killer application. Used to describe a game/application that really makes a system/computer.

"An application that proves the usefulness of the underlying technologies"

Defined by Jawed Karem in the speech "YouTube: From Concept to Hyper-growth" held at University of Illinois at Urbana-Champaign ACM Conference, Oct 21, 2006. (www.youtube.com/watch?v=nssfmTo7SZg)