noun
private bath (at an onsen, etc.)
A bath that can be reserved for private use, typically at a hot spring inn or public bath facility. Often used in contrast to shared baths.
See also: 貸切風呂
この旅館には貸切湯が三つあります。
This inn has three private baths.
家族で貸切湯を予約した。
We reserved a private bath for the family.
貸切風呂 is a more general term for a private bath, not limited to hot springs. 貸切湯 specifically implies a hot spring bath (湯).
Compound of 貸切 (reserved, private) and 湯 (hot water, bath). The term is straightforward and modern.