Killer application. Tech industry jargon for an app so obviously massively useful to someone that they will rush out and buy it immediately, even purchasing the hardware need to run the app, if necessary. In 1979, VisiCalc was considered the first killer app and helped drive sales of the Apple II, the only computer that could run the software. In 1996, Super Mario 64 was the killer app that brought players to the Nintendo 64 gaming console. Today killer apps tend to run on most devices.