The name of the corporation that runs a large research facility on Phobos (one of the moons of Mars) in the video game Doom. They do lots of experiments with nasty things like inter-dimensional gateways, and something that seems to involve having lots of toxic nuclear waste sitting around in open pools (the third level of Doom is called "Toxin Refinery", in fact). I have seen some references which claim that UAC is an acronym for "United Aerospace Corporation", but I'm not completely sure this is true.
UAC's logo pervades the first episode of Doom (Doom is divided into episodes, which are subdivided into levels). Pretty much everywhere you look, you'll see their logo on the floor or walls.
Whoever did the interior design for UAC's Phobos base should be fired ASAP. They seemed to be big fans of hidden rooms, forcing people to cross pools of glowing slime to hit switches, and also pentagrams. Thankfully, they also helpfully stocked the base full of shotguns, ammo, and the occasional BFG9000 (for all your demon-killing needs!).
UAC is probably looking at a very substantial class action lawsuit after their experiments get out of hand, and (later on) result in almost the complete decimation of Earth.