hidden room; secret chamber
A room concealed from view or kept secret, often for privacy, security, or illicit purposes.
There seems to be a hidden room in this old mansion.
The spies held a secret meeting in the hidden room.
Compound of 隠し (kakushi, 'hiding') + 部屋 (heya, 'room'). The exact historical derivation is uncertain, but the word follows a common pattern for describing hidden spaces.