steam bath; sauna
A bath that uses steam rather than hot water; often contrasted with 蒸し風呂. The kanji 空 (empty) may be misleading, as the bath is filled with steam.
See also: 蒸し風呂
I relaxed and sweated in the steam bath at the inn.
empty bath; unfilled bathtub
A bathtub that has no water in it; the literal reading of the kanji. Less common than the steam bath sense.
I fill the empty bathtub with water.
Compound of 空 (から, empty) and 風呂 (ふろ, bath). The steam bath sense likely derives from the idea of a bath filled only with steam, not water.