Doom (typeset as DOOM in official documents) is a 1993 science fiction horror-themed first-person shooter (FPS) video game by id Software. It is considered one of the most significant and influential titles in the video game industry, for having ushered in the popularity of the first-person shooter genre. The original game was divided into three nine-level episodes and was distributed via shareware and mail order. The Ultimate Doom, an updated release of the original game featuring a fourth episode, was released in 1995 and sold at retail.
In Doom, players assume the role of an unnamed space marine, who became popularly known as "Doomguy", fighting his way through hordes of invading demons from Hell. With one third of the game, nine levels, distributed as shareware, Doom was played by an estimated 15–20 million people within two years of its release, popularizing the mode of gameplay and spawning a gaming subculture. In addition to popularizing the FPS genre, it pioneered immersive 3D graphics, networked multiplayer gaming, and support for customized additions and modifications via packaged files in a data archive known as "WADs". As a sign of its effect on the industry, first-person shooter games from the genre's boom in the 1990s, helped in no small part by the game's release, became known simply as "Doom clones". Its graphic violence, as well as satanic imagery, made Doom the subject of controversy.
The Doom franchise was later continued with the follow-up Doom II: Hell on Earth (1994) and numerous expansion packs, including Master Levels for Doom II (1995), and Final Doom (1996). Originally released for PC DOS, the games have later been ported to numerous other platforms. Once the game's source code was released in 1997, it spawned even more adaptations, as fans further ported the code to countless devices. The series started to lose mainstream appeal as the technology of the Doom game engine was surpassed in the mid-1990s, although fans have continued making WADs, speedruns, and modifications to the original. The franchise again received popular attention in 2004 with the release of Doom 3, a retelling of the original game using new technology, and an associated 2005 Doom motion picture. Doom 4 was announced as in production in 2008 and was later retitled simply as Doom