1.
locked room; sealed room; closed room
A room that is sealed or locked from the inside, especially in the context of mystery fiction where a crime is committed in a seemingly impossible setting.
2.
behind closed doors; in secret
Used figuratively to describe decisions or discussions conducted in secret, without public scrutiny or transparency.
Composed of 密 (secret, closed) + 室 (room). Has two distinct usage areas: mystery fiction and politics.
Mystery fiction
The 密室ミステリー (locked-room mystery) is a beloved genre in Japan, with classic examples by Edogawa Ranpo and others.
Political use
Carries a negative connotation of secrecy and lack of transparency.