A killer application (commonly shortened to killer app) is a computer program that is so useful that people will buy a particular computer hardware, gaming console, and/or an operating system simply to run that program.

The first example of a killer app is generally agreed to be the VisiCalc spreadsheet on the Apple II platform. The machine was purchased in the thousands by finance workers (in particular, bond traders).

The next example is another spreadsheet, Lotus 1-2-3. Sales of IBM's PC had been slow until 1-2-3 was released, but only months later it was the best selling computer.

A killer app can provide an important niche market for a non-mainstream platform. Aldus PageMaker and Adobe PostScript gave the graphic design and desktop publishing niche to the Apple Macintosh in the late 1980s, a niche it retains to this day despite PCs running Windows having been capable of running versions of the same applications since the early 1990s.

There have been a number of new uses of the term. For instance the usefulness of e-mail drew many people to use the Internet, while the Mosaic web browser is generally credited with the initial rapid popularity of the World Wide Web. The term has also been applied to video games that cause consumers to buy a particular video game console to play them; see killer game.

Computer experts sometimes use the phrase with reference to other technologies to explain its significance to laypersons. A set of these analogies includes:

• the telephone (microphone and earphone) - talking to neighbors via a telephone exchange
• the (single-expansion) steam engine - railway transport (although its factory use was of prior significance)
• the triple-expansion steam engine - steamships
• the steam turbine - the HMS Dreadnought
• rubber - the pneumatic tire, or raincoats
• the gasoline engine - the automobile (though motorboat "one-lunger" engines were the first widespread sales)