What is a 4x board game?

Excuse me, could you please elaborate on the concept of a "4x board game"? I'm not entirely familiar with the term and would appreciate a concise explanation. Are we referring to a specific type of strategy board game that incorporates certain elements or mechanics? What are the defining characteristics of a 4x board game, and how do they differentiate it from other board game genres? Additionally, could you provide any examples of popular 4x board games that might help me better understand the concept? Thank you in advance for your clarification.