noun
indoor hot-spring bath
Refers to a bath inside a building that uses natural hot-spring water, as opposed to an outdoor bath (露天風呂).
この旅館は内湯と露天風呂の両方がある。
This inn has both an indoor hot-spring bath and an outdoor bath.
露天風呂 is an outdoor bath, while 内湯 is an indoor bath.
Compound of 内 (uchi, 'inside') and 湯 (yu, 'hot water').