A shallow bath in a bathhouse or hot spring designed for reclining, often with a headrest.
I relaxed in the lie-down bath at the hot spring.
This facility has a lie-down bath and an open-air bath.
Compound of 寝 (ne, 'sleep' or 'lie down') and 湯 (yu, 'hot water'). The exact historical derivation is uncertain, but the term straightforwardly describes a bath for reclining.