The Jargon File (version 4.4.7, 29 Dec 2003):

killer app


    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??