eXplore, eXpand, eXploit, eXterminate

It also refers to the potential minimum number of eX-spouses you will have after you start playing.   

Sorry, that was bad...   

I am thankful my wife allows me to indulge my gaming hobby.

I think this person would like to have a serious answer instead of weird sex references.

But he already got a serious answer. The weird sex references were just a bonus.

LOL!

@dragonrift: As V1m pointed out, 4x stands for "eXplore, eXpand, eXploit, eXterminate". It's a simple phrase that refers to the 4 main stages of most types of space conquest games (along with other strategy games like Civ).

1 - eXplore) The first stage of the game, where you're sending out scout ships and survey vessels to find new planets to colonize and new anomalies to explore.

2 - eXpand) The second stage, where you deploy colony ships to settle the new worlds you've found.

3 - eXploit) You exploit the resources of the planets you've settled, using them to build up your infrastructure on your colonies, researching technology and building a powerful fleet.

4 - eXterminate) The final phase of the game, where you go out and conquer and/or destroy all those who oppose you and stand in the way of victory!

That, in a (slightly expanded) nutshell is what 4x means.

Learning is fun!

Well I reckon you picked a pretty good introduction to the genre. I've been playing 'em since Reach For The Stars on the Amiga... I gotta warn you - once you're hooked, you're hooked ;o)