Why are they called 4X games?

Excuse me, I'm a bit confused about the terminology. Could you please explain why these types of games are referred to as '4X games'? Is it because they involve some sort of quadruple experience or does the name stem from a specific feature or mechanic that's common across these games? I'd appreciate it if you could clarify this for me and give me a brief overview of the key elements that define a 4X game.