A software application that is exceptionally useful or exciting. Killer apps are innovative and often represent the first of a new breed, and they are extremely successful. For example, in the late 1970s, the VisiCalc spreadsheet was the killer app for the Apple II, providing reason enough to purchase the hardware. VisiCalc was followed by Lotus 1-2-3 and Excel for the IBM PC. In the early 1990s, the Mosaic Web browser was the killer app for the Internet. It was quickly followed by Netscape and others. Contrast with filler app.