noun
confinement room; detention room; brig
A room for holding someone against their will, such as a prison cell, a detention room in a police station, or a brig on a warship.
容疑者は警察署の監禁室に入れられた。
The suspect was placed in the police station's detention room.
軍艦の監禁室に水兵が閉じ込められた。
The sailor was locked in the warship's brig.
留置場 is a police detention facility, often larger than a single room, whereas 監禁室 can refer to a single room.
Compound of 監禁 (confinement) + 室 (room).