A 4X game is one where players control a country or an empire and are expected to:
- eXplore the game world
- eXpand your empire by creating new cities/colonies etc.
- eXploit the resources in your empire
- eXterminate rival empires by attacking and eliminating them
The term was first applied to Master of Orion, and is most commonly associated with the Civilzation series. Every game in the Europa Universalis series is considered a 4X game.