Singular
dressing room

Plural
dressing rooms

  1. (countable) A dressing room is a place in a theatre, movie studio, television studio, etc. where a performer changes from his street dress into a costume. A locker room is sometimes called a dressing room.
    He put on his costume and make up in a dressing room on the lower level of the theatre.
    Her dressing room was crowded with adoring fans after every performance.