Doom is a first-person shooter game developed and published by id Software. Released on December 10, 1993, for DOS, it is the first installment in the Doom franchise. The player assumes the role of a space marine, later unofficially referred to as Doomguy, fighting through hordes of undead humans and invading demons. The game begins on the moons of Mars and finishes in hell, with the player traversing each level to find its exit or defeat its final boss. It is an early example of 3D graphics in video games, and has enemies and objects as 2D images, a technique sometimes referred to as 2.5D graphics.

Doom was the third major independent release by id Software, after Commander Keen (1990–1991) and Wolfenstein 3D (1992). In May 1992, id started developing a darker game focused on fighting demons with technology, using a new 3D game engine from the lead programmer, John Carmack. The designer Tom Hall initially wrote a science fiction plot, but he and most of the story were removed from the project, with the final game featuring an action-heavy design by John Romero and Sandy Petersen. Id published Doom as a set of three episodes under the shareware model, marketing the full game by releasing the first episode free. A retail version with an additional episode was published in 1995 by GT Interactive as The Ultimate Doom.

Doom is a first-person shooter presented with 3D graphics. While the environment is shown in a 3D perspective, the enemies and objects are instead 2D sprites rendered at fixed angles, a technique sometimes referred to as 2.5D graphics or billboarding. In the single-player campaign mode, the player controls an unnamed space marine—later unofficially termed “Doomguy”—through military bases on the moons of Mars and in hell. To finish a level, the player must traverse through labyrinthine areas to reach a marked exit room. Levels are grouped into named episodes, with the final level of each focusing on a boss fight.