Bathing was the act of self-cleaning and maintaining one's personal hygiene, that often had relaxing effects. For humanoids, this was often done in a bath tub or shower, using water; or in a sonic shower. (TOS: "The Naked Time", Star Trek: The Motion Picture, VOY: "Caretaker")
Some cultures used mud baths or protein baths as a method of relaxing. (TNG: "Birthright, Part I", "Cost Of Living", DS9: "Who Mourns for Morn?")
Bathing could take place in a bathroom, or a specialized establishment such as the Hoobishan Baths on Trill. (DS9: )