An unoccupied or vacant room; commonly used in contexts like rentals or hotels to mean an available room.
駅の近くで家賃が安くて条件の良い空室を見つけた。
I found a vacant apartment near the station with low rent and good conditions.
ホテルに空室はありますか？
Do you have any vacant rooms at the hotel?
今期は空室率が下がり、ほとんど満室になった。
This season the vacancy rate fell, and rooms are almost fully booked.