4X refers to a genre of strategy video game with four primary goals: eXplore, eXpand, eXploit, and eXterminate. A 4X game can be turn-based or real-time. Perhaps the best-known example of this genre is Sid Meier’s Civilization series.
4X games often cover a very large period of time, giving the player the control of an entire civilization or species. Typically these games have a historical setting, encompassing a large amount of human history (Empire Earth, Civilization, Golden Age of Civilizations) or a science fiction setting where the player controls a species set to dominate the galaxy (Master of Orion, Galactic Civilizations).