Phobos is the larger and innermost of the two moons of the planet Mars, the second being Deimos. It is the scene of the first Doom episode, Knee-Deep in the Dead. "Phobos" is the name of a god in Greek mythology and it can be translated as "panic fear", "flight" or "battlefield rout".
In Doom, Phobos is depicted with Earth-like gravity, a thick atmosphere, and having tall, seemingly vegetation-covered mountains; the sky texture for the episode was derived from a photograph  taken of Yangshuo Cavern in China.
In reality, Phobos is a rock 22 kilometers in diameter with gravity less than a thousandth of that on Earth, and no atmosphere (even if an atmosphere could be generated artificially, the gravity would be insufficient to hold it in place). Phobos' gravity is so weak that a human being could escape it by jumping. In order to be more plausible, Doom 3 moved the plot to Mars.
According to information on an official Nintendo webpage for Doom 64, the initial part of the game occurs on Phobos, a detail also consistent with some pre-release reviews of the game whose canonicity was otherwise unverified. The game itself was vague about its setting, referring only to a "planetary quarantine," which has sustained speculation about the intended setting since the game's original release.
In Doom 3, Phobos serves as an orbital communications relay station via the Phobos Routing Station Uplink.
In Doom Eternal, Phobos is the starting location for the Mars Core mission. The Doom Slayer travels to a mining and communication relay in one of Phobos' craters and gains control of its BFG-10000 weapon, using it to blast a hole in the surface of Mars. In the game, the asteroid seems to maintain some level of atmosphere, since mist can be seen extending out into hilly terrain in the distance. This facility also served as one of the scenes during the QuakeCon 2018 gameplay reveal. It featured the orbital base under attack by demons, along with the BFG-10000 firing periodically towards an unconfirmed location.
Phobos serves as the setting for the 2019 film Doom: Annihilation, which retells the story of Knee-Deep in the Dead with various artistic liberties. Rather than being linked with Deimos as in the original game, the Phobos teleportation lab is directly linked to Earth via a gate left on the moon by an unknown ancient civilization. Phobos is stated in the film to be a highly unstable location often referred to as the "Doomed moon."