A changing room, locker room, dressing room or changeroom (regional use) is a room or area designated for changing one's clothes. Changing rooms are
Both exist. Fitting rooms are in department stores; dressing rooms are in theatres.
"Changing room" is a more common term than "fitting room": Source: Google Trends - United States, 2004 - present But part of the reason for
You can use both changing room or fitting room if you are in a shop. But if you are at the gym, you'll only say changing.