waiting room; anteroom; green room
A room where people wait before an appointment, performance, or meeting. 控室 is a common abbreviated spelling.
The performers are waiting in the green room.
面接の前に控え室でしばらく待たされた。
I was made to wait for a while in the waiting room before the interview.
I waited in the green room until it was my turn.
Before the interview, I was shown to a waiting room.
The players entered the waiting room before the match.