noun
A shallow bath in a bathhouse or hot spring where you can lie down and relax. Often found in onsen facilities.
See also: 寝湯
旅館の寝転び湯で、のんびりと星を眺めた。
I relaxed and gazed at the stars in the lie-down bath at the inn.
寝湯 is a more general term for a bath where you can lie down, while 寝転び湯 emphasizes the act of lying down and rolling over.
Compound of 寝転び (nekorobi, 'lying down') + 湯 (yu, 'hot water, bath').