noun
next room; adjoining room
Refers to a room directly next to or connected to the current one, often in a house, apartment, or building.
隣室から音楽が聞こえてきた。
I could hear music coming from the next room.
彼は隣室で休んでいる。
He is resting in the adjoining room.
隣 is a more general word for 'next door' or 'neighboring' and can refer to a house, building, or seat, while 隣室 specifically means the next room.
Compound of 隣 (りん, 'neighboring') and 室 (しつ, 'room'). The reading りんしつ is the standard on'yomi combination.