"X-COM is a series of computer games, started by Mythos Games and MicroProse in 1993. The first three titles were single-player tactical games while the fourth was an action-based space combat/strategy game.
The first installment, UFO: Enemy Unknown in Europe (also known as X-COM: UFO Defense in North America) was written by a team led by Julian Gollop. MicroProse quickly had an internal team create the sequel X-COM: Terror from the Deep. Subsequently, the Gollop brothers went straight to work on X-COM: Apocalypse, which would end up being the third in the series when released in 1997. Soon after Apocalypse, MicroProse was bought by Hasbro Interactive, which released two more X-COM games."
Source: Wikipedia, "X-COM", available under the CC-BY-SA License.