UAC

The Union Aerospace Corporation (UAC) is a fictional military-industrial conglomerate that operates the radioactive waste facilities on Mars, as well as on its two moons Phobos and Deimos where Doom takes place. The military, UAC's biggest client, used the latter two facilities to conduct experimental inter-dimensional travel through teleporters. This project grew unstable, and project "volunteers" began to suffer from a violent form of progressive insanity culminating in sudden, pyrotechnic death. The story begins when distress messages sent from the moons reach Mars, where the player is stationed.

In the Doom Bible, the organization was called UAAF (United Aerospace Armed Forces) and operated on Tei Tenga.

UAC in Doom and Doom II
Doom 1 placed the UAC bases on Mars and its two moons Deimos and Phobos. Little detail was given about UAC's activities given the sketchy nature of these games' plot, but it was made clear that the UAC had been researching technology used for teleportation and was a major contractor with the military, probably the reason for their bases being guarded by Marines (of which the player is one).

UAC in Doom 3
With the retelling to the first Doom, the game received a large overhaul, greatly developing the story. Increased focus was placed on the importance of the UAC research being conducted (specifically research under the guidance of Malcolm Betruger) by the Delta labs division. A great deal of the UAC’s research seems to be focused on teleportation, and experimentation with species retrieved from beyond the teleport.

UAC in Doom movie
The UAC has a role in the Doom movie, but their ambitions and scientific work deviate from the games, as they focus on genomic research instead of teleportations.

It should be noted, however, that genomic research was mentioned in Doom 3.

UAC website
For the promotion of Doom 3, id Software created a fake UAC website. The "trajectory" mentioned on the site has two meanings. Within the context of the game, it represented the estimated arrival time to Mars of the shuttle that carried the Marine, Swann and Campbell. In real life, it was actually a countdown to the game's release date. (The site now redirects to the id Software website.)

Recently the Doom RPG game for cellphones was released. On the main Doom RPG website there are links to what seems to be a more realistic Union Aerospace Corporation website. Many of the links lead to Access Denied pages. It holds some more information on the history of the UAC.