hot spring symbol; onsen mark
The ♨ symbol used on maps and signs to indicate a hot spring or public bath.
When I spot the hot spring symbol on a map, I immediately want to go there.
traditional love hotel
Slang from the Showa era, referring to a love hotel that used the ♨ symbol as a discreet sign. Now largely historical.
See also: 連れ込み宿 · 逆さクラゲ (さかさくらげ)
In the Showa period, the hot spring symbol was also used as a sign for traditional love hotels.