It is the simple designation of a savage humanoid alien given by two separate rocket crews to Mars, both of which see each other killed by the creature, in the first instance almost to a one. It is the antagonist of the 1958 feature film, It! The Terror from Beyond Space. Both the look of the creature and the claustrophobia of the enclosed rocket that becomes an inescapable stalking ground were said by Ridley Scott to be huge influences on the 1979 feature film Alien. The script was written by Jerome Bixby, author of The Twilight Zone classic It's A Good Life whose premise he contributed to the original Star Trek episode Charlie X. It is likely his thoroughness that led to such offhand references as the ship having artificial gravity, when small plot points like that were often ignored by lesser films of the era.
It is the future year (for 1958, anyway) 1973 and a rocket crew has landed on Mars to arrest and bring home for trial the only survivor of the prior expedition, Colonel Edward Carruthers, who stands accused of murdering his crewmates for their supplies, panicking into treachery when rescue seemed uncertain. His claims of an unknown creature having killed his friends are dismissed as either a cold man either trying to save his skin, or a good man who has done something horrid and gone mad as a result. Carruthers is given free run of the ship on its return to Earth - there is simply no where for him to run, and sabotaging the ship will doom him as well. Liftoff is briefly delayed by an opened hatch left unattended, but this is sealed in no time.
The crew varies between treating Carruthers as a man innocent until proven guilty and avoiding him as a murderous monster spinning tall tales for the gullible. For his part, he keeps calm, resigned to the fact that he has no way of proving his claims. The commander of the recovery mission sees that this man is not a true killer, but resents his not owning up to what he sees as his panic-driven crimes. This debate rapidly becomes moot when crewmembers begin to vanish and then turn up dead, drained of all water, and at times and places Carruthers was nowhere near as well. A shadowy figure, the odd conditions of the corpses, and inhuman noises from the air ducts exonerate Carruthers, but there is no joy for him or anyone else in being proven right. The hunt begins for a creature that is very strong and durable, and capable of traversing the ship's air ducts to stalk and store its prey.
At some point, they speculate that It may be a mutated survivor of whatever civilization barren Mars once held, perhaps destroyed by radiation from a war, and in need of something Mars has barely any of : water. This is somewhat backed up when It proves as resistant to radiation as to gunfire. Efforts to booby-trap or proof entry points against the creature's passage prove pointless. As a last-ditch effort, Carruthers gathers the survivors on the command deck, and has them all don their spacesuits. Despite the creature's lung capacity, it is suffocated when they open the airlock while breaking through to their position.
Carruthers is of course cleared, but the price is higher than merely the lives lost among the two crews. Earth it seems, will be forced for a long while to keep Mars out of its exploration equations.