noun
public bath with mixed bathing
A traditional public bath where men and women bathe together. Often associated with older hot-spring resorts. Compare 混浴 (mixed bathing in general).
See also: 混浴
この温泉には昔ながらの入り込み湯が残っている。
This hot spring still has an old-fashioned mixed public bath.
混浴 refers to mixed bathing in general, while 入り込み湯 specifically means a public bath facility where mixed bathing takes place.
Compound of 入り込み (entering, from 入り込む) and 湯 (hot water, bath). The exact historical derivation is uncertain, but the term likely originated in hot-spring areas where communal mixed bathing was common.