A hot spring was a spring produced by the emergence of geothermally heated groundwater from a planet's crust.
During a trip to Risa in 2373, Quark suggested that he, Julian Bashir and Jadzia Dax should go skinny dipping in the Risian hot springs. (DS9: "Let He Who Is Without Sin...")
In 2376, Tuvok said it was theoretically possible for L class planetoids to contain hot water springs that could sustain a stranded individual for several weeks. (VOY: "Riddles")