What is the 51st state in the United States?

Excuse me, I'm curious about a topic related to the United States' political geography. Could you please clarify for me what exactly is meant by the phrase "the 51st state in the United States"? As of my last knowledge, there are 50 states in the US. Is there a new state that has been recently added to the union, or is this a reference to a potential future state, or perhaps a colloquial term for a territory or district that enjoys a special status? I'd appreciate any clarification you could provide.