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 include:
- The 1980 Atari VCS port of the arcade game Space Invaders quadrupled sales of the three-year-old console.
- Star Raiders, released in 1979, may have been a system-seller for the Atari 400/800 computers. Another was Eastern Front (1941).
- In 1996, Computer Gaming World wrote that Wizardry: Proving Grounds of the Mad Overlord (1981) "sent AD&D fans scrambling to buy Apple IIs".
- 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 NES 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 NES 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.
- In the early lifespan of the original Sony Playstation, games like Wipeout, Ridge Racer, Tomb Raider, Resident Evil and NFL GameDay were considered its killer apps.[a]
- Euros 96 was a major system-seller for the Sega Saturn in the United Kingdom.
- Sega Rally Championship was another major system-seller for the Sega Saturn in the United Kingdom, becoming the fastest selling CD game at the time. 
- Die Hard Arcade and Fighters Megamix boosted the Sega Saturn's sales in the United States.
- Virtua Fighter and Sakura Wars were considered crucial to the Sega Saturn's success in Japan.
- Super Mario 64, and GoldenEye 007 could be considered killer apps for the Nintendo 64.
- Virtua Fighter 3 was the killer app for the Sega Dreamcast in Japan.
- Sonic Adventure was hyped as being the Sega Dreamcast's main title.
- NFL 2k can be considered a killer app for the Sega Dreamcast in North America. 
- Gran Turismo 3 is considered the killer app for the Sony PlayStation 2.
- 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.
- Many video game and technology critics point to Xbox Live as a more general "killer app" for the original Xbox console.
- Blue Dragon served as a killer app for the Xbox 360 in Japan.
- Metal Gear Solid 4 caused a boost in PlayStation 3 sales.
- Super Mario Galaxy and its sequel were acclaimed as killer apps for Wii. 
- Bloodborne was known as the first killer app for the PlayStation 4 console.
- The Legend of Zelda: Breath of the Wild was the killer app for the Nintendo Switch console.
- 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.
- AmigaOS: Deluxe Paint, Video Toaster
- Mac OS: Microsoft PowerPoint (before 1990)
- RISC OS: Sibelius
- The World Wide Web: the web browsers Mosaic and Netscape Navigator; the music-sharing program Napster
- While Tomb Raider was also released to MS/DOS and Sega Saturn and Resident Evil was later released for Saturn and Windows, both games contributed substantially in the original Playstation's early success. (see Blache Fabian & Lauren Fielder and NG Alphas)
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- Scannell, Ed (February 20, 1989). "OS/2: Waiting for the Killer Applications". InfoWorld. Vol. 11 no. 8. Menlo Park, CA: InfoWorld Publications. pp. 41–45. ISSN 0199-6649. Early use of the term "Killer Application".
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- Earliest usage cited in Oxford English Dictionary
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And if there was one game that sold Playstation on launch, it was WipEout
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Virtua Fighter's 3D characters have a presence that 2D sprites just can't match. The characters really do seem 'alive', whether they're throwing a punch, unleashing a special move or reeling from a blow ... The Saturn version of Virtua Fighter is an exceptional game in many respects. It's arguably the first true 'next generation' console game, fusing the best aspects of combat gameplay with groundbreaking animation and gorgeous sound (CD music and clear samples). In the arcades, Virtua Fighter made people stop and look. On the Saturn, it will make many people stop, look at their bank balance and then fork out for Sega's new machine. Over to you, Sony.
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Many composers bought an Archimedes simply to have access to the program.
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