dressing room

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A room, as in a theater, for changing costumes or clothes and applying makeup.

from The Century Dictionary.

• noun A room, as one opening from a bedroom, intended to be used for dressing: as, the dressing-rooms of a theater.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A room in a theatre etc in which performers may change costume and apply makeup
• noun A small room adjoining a domestic bedroom where people may dress or undress in privacy
• noun A changing room

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a room in which you can change clothes