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1.
stands for killer application. Used to describe a game/application that really makes a system/computer.
Halo was the killer app for the Xbox
by where am i? March 13, 2005
 
2.
"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)
A typical killer application:
- the first spreadsheet application

Killer apps in the internet domain:
- Wikipedia
- LiveJournal
- MySpace
- Flickr
- YouTube
by Papy Jojo March 19, 2007