4X refers to a video game genre with four primary goals, namely to "eXplore, eXpand, eXploit, and eXterminate." The first 4X games were turn-based, but game developers have created real-time 4X games, as well.
The term "4X" was first used by Alan Emrich, in the September 1993 issue of Computer Gaming World, in the review of Master of Orion. Mainstream reviewers now use "4X" in articles about games which follow a similar pattern.
Many typical features of 4X games had appeared earlier, in board games and in 1970s computer games.
The HomeWorld series is probably an rts.