In marketing terminology, a killer application (commonly shortened to killer app) is any computer program or software that is so necessary or desirable that it proves the core value of some larger technology, such as computer hardware, a video game console, software, a programming language, a software platform, or an operating system. In other words, consumers would buy the (usually expensive) hardware just to run that application. A killer app can substantially increase sales of the platform on which it runs.
One mark of a good computer is the appearance of a piece of software specifically written for that machine that does something that, for a while at least, can only be done on that machine.
One of the first recognized examples of a killer application is generally agreed to be the VisiCalc spreadsheet for the Apple II series. Because it was not available on other computers for 12 months, people spent $100 for the software first, then $2,000 to $10,000 on the Apple computer they needed to run it. BYTE wrote in 1980, "VisiCalc is the first program available on a microcomputer that has been responsible for sales of entire systems", while Creative Computing's VisiCalc review was subtitled "reason enough for owning a computer". Others also chose to develop software, such as EasyWriter, for the Apple II first because of its increasing sales.
Lotus 1-2-3 similarly benefited sales of the IBM PC. Noting that computer purchasers did not want PC compatibility as much as compatibility with certain PC software, InfoWorld suggested "let's tell it like it is. Let's not say 'PC compatible,' or even 'MS-DOS compatible.' Instead, let's say '1-2-3 compatible.'" Another killer app is WordStar, the most popular word processor during much of the 1980s.
The UNIX Operating System served as a killer application for the DEC PDP-11 minicomputer and VAX-11 minicomputer during roughly 1975–1985. Many of the PDP-11 and VAX-11 processors never ran DEC's operating systems (RSTS or VAX/VMS), but instead, they ran UNIX, which was first licensed in 1975. To get a virtual-memory UNIX (BSD 3.0) you had to purchase a VAX-11 computer. Many universities wanted a general-purpose timesharing system that would meet the needs of students and researchers (early versions of UNIX included free compilers for C, Fortran, and Pascal; at the time, offering even one free compiler was unprecedented). From its inception UNIX could drive high-quality typesetting equipment and later PostScript printers using the nroff/troff typesetting language, and this was also unprecedented for its time. UNIX was the first operating system offered in source-license form (a university license cost only $10,000, less than a PDP-11), allowing it to run on an unlimited number of machines, and allowing the machines to interface to any type of hardware because the UNIX I/O system was extensible.
The first recorded use of the term in print was 1988, in PC Week 24 May. 39/1. "Everybody has only one killer application. The secretary has a word processor. The manager has a spreadsheet."
The definition of "killer app" came up during Bill Gates's questioning in the United States v. Microsoft Corp. antitrust case. Bill Gates had written an email in which he described Internet Explorer as a killer app. In the questioning, he said that the term meant "a popular application", and did not connote an application that would fuel sales of a larger product or one that would supplant its competition, as the Microsoft Computer Dictionary defined it.
Introducing the iPhone in 2007, Steve Jobs said that "the killer app is making calls." Reviewing the iPhone's first decade, David Pierce for Wired wrote that although Jobs did indeed prioritize a good experience making calls in the phone's development, other features of the phone soon turned out to be more important, such as its data connectivity and ability to install third-party software (which was added later).
The term has also been applied to computer and video games that persuade consumers to buy a particular video game console or other video game hardware product over a competing one, by virtue of being exclusive to that platform. Such a game is also known in video game parlance as a "system seller". Examples of video game killer applications are:
- Star Raiders, released in 1979, was considered to be the killer app for the Atari 400/800 computers. Another was Eastern Front (1941).
- The 1980 Atari VCS port of the arcade game Space Invaders quadrupled sales of the three-year-old console.
- In 1996, Computer Gaming World wrote that Wizardry: Proving Grounds of the Mad Overlord (1981) "sent AD&D fans scrambling to buy Apple IIs".
- A port of Donkey Kong was the killer app for the ColecoVision console in 1982.
- The Famicom home port of Xevious is considered the console's first killer app, which caused system sales to jump by nearly 2 million units.
- The video game website GameTrailers considers the Super Mario Bros. games to be the killer app for nearly all Nintendo home consoles, Tetris as the killer app for the Game Boy, Grand Theft Auto III for the PlayStation 2, Super Smash Bros. Melee for the Nintendo GameCube, and Wii Sports for the Wii.
- Computer Gaming World stated that The Legend of Zelda on the Nintendo Entertainment System, Phantasy Star II on the Sega Genesis, and Far East of Eden for the NEC TurboGrafx-16 were killer apps for their consoles.
- John Madden Football's popularity in 1990 helped the Genesis gain market share against the Super Nintendo in North America.
- Sonic the Hedgehog, released in 1991, was hailed as a killer app as it revived sales of the (by then) three-year-old Genesis. 
- Street Fighter II, originally released for arcades in 1991, became a system-seller for the Super Nintendo when it was ported to the platform in 1992.
- Myst and The 7th Guest, both released in 1993, drove adoption of CD-ROM drives for personal computers.
- Pokémon Red and Blue could be called a "killer app" for the seven-year-old Game Boy.
- Super Mario 64, GoldenEye 007 and The Legend of Zelda: Ocarina of Time could be considered killer apps for the Nintendo 64.
- Wii Sports was the killer app for the Wii because it was packaged with the console and attracted many non-traditional purchasers in the console's early years.
- Tekken 5 and Soulcalibur III, both developed by Namco, were exclusively available for the PlayStation 2.
- Final Fantasy VII became a killer app for the Sony PlayStation console.
- Super Mario Galaxy and its sequel were acclaimed as killer apps for Wii. 
- Halo: Combat Evolved and Halo 2 are considered the killer apps for the original Xbox, and the subsequent series entries went on to become killer apps for the Xbox 360 and Xbox One.
- Gears of War and Uncharted were prominent killer apps for Xbox 360 and PlayStation 3, respectively.
- Bloodborne was known as the first killer app for the PlayStation 4 console.
- The Legend of Zelda: Breath of the Wild was considered a killer app as a launch title for the Nintendo Switch.
- Half-Life: Alyx was considered a killer app for virtual reality headsets, on account of being thought of by many as the first true AAA virtual reality game. Sales of VR headsets — such as the Valve Index — increased dramatically after its announcement, suggesting users bought the product specifically for the game.
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- Earliest usage cited in Oxford English Dictionary
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Many composers bought an Archimedes simply to have access to the program.
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