a room (an enclosed space in a building). Also frequently used as a suffix meaning 'room' (e.g. 教室, 会議室).
会議室は三階にあります。準備ができたらお知らせください。
The conference room is on the third floor. Please let me know when you're ready.
図書室では静かに勉強してください。
Please study quietly in the library (reading room).
寝室の窓から夜景がきれいに見える。
You can see a beautiful night view from the bedroom window.
rare/archaic: the wife of a person of high rank (an old term; rarely used today).
古い史料には、将軍の室が祭祀を取り仕切ったと記されている。
Old records state that the general's wife oversaw certain rites.
rare/archaic: a scabbard or sheath for a sword (an old usage).
古文には『刀は室に収められた』とあり、ここでの室は鞘を指す。
In old writings it says 'the sword was placed in the 室,' where 室 refers to the scabbard.
astronomy: the name of one of the Chinese 'Twenty-Eight Mansions' (the mansion called 'Shi' / 室).
二十八宿の一つ、室は玄武に関係する星宿の一部である。
Shi (室) is one of the Twenty-Eight Mansions and is part of the constellation group associated with Xuanwu.