Definitions for killer app
Random House Webster's College Dictionary
a computer application that surpasses its competitors.
Origin of killer app:
A killer application.
The New Hacker's Dictionary
The application that actually makes a sustaining market for a promising but under-utilized technology. First used in the mid-1980s to describe Lotus 1-2-3 once it became evident that demand for that product had been the major driver of the early business market for IBM PCs. The term was then retrospectively applied to VisiCalc, which had played a similar role in the success of the Apple II. After 1994 it became commonplace to describe the World Wide Web as the Internet's killer app. One of the standard questions asked about each new personal-computer technology as it emerges has become “what's the killer app?”