noun
A room equipped with lockers, typically in gyms, schools, or workplaces.
ジムのロッカー室で着替えた。
I changed clothes in the gym's locker room.
更衣室 is a general changing room, while ロッカー室 specifically emphasizes the presence of lockers.
Compound of ロッカー (from English 'locker') and 室 (room).