Game series, based off of UFO lore as well as own made up stuff. Original came out in 1994, with Terror from the Deep coming out in 1995, and Apoclypse in 1997 as direct descendents. There are other games based in the same universe, as well as two planned but cancelled games named Genesis and Alliance.
PC Gamer has the original free in its 2000 september disc.
As a word it can be used to describe any type of alien defense force, as X-Com comes from a 3 word phrase of close meaning.
You think most X-Coms are pretty crazy, don't you?