MC in the context of BTS usually stands for Master of Ceremonies, essentially the host of an event. While no single member holds the permanent title of "MC" for BTS, all the members have taken on the role of MC at various points in their careers.
Essentially, "MC" signifies the member(s) who are taking the lead in guiding or presenting an event, show, or segment. This could include introducing performances, interviewing guests, leading discussions, or keeping the energy high during concerts or fan meetings.
Therefore, instead of a fixed role, it's a function that any BTS member can and does fulfill.
