1.
changing room; locker room; dressing room
A room where people change clothes, found in gyms, pools, sports facilities, and stores.
更衣室 is the room where you change into or out of sports clothes, a uniform, or a swimsuit. Formation: 更衣 'changing clothes' + 室 'room'.
Where you find one: a ジム, a プール, スポーツ施設 of any kind, and the staff areas of workplaces where a uniform is worn.
Collocations
Do not use it of a shop: trying on clothes before buying happens in a 試着室 (fitting room). A 控室 is a room to wait in, not to change in.
Related Words
Related:
試着室 (fitting room (in a shop))
Related:
控室 (waiting room)
Related:
着替える (to change clothes)
Related:
脱衣所 (changing room)