Bathing is putting the body into a fluid, most often water, or a solution with water. Usually it is done for hygiene or for religious purposes, or for fun. Sometimes bathing is also done as a form of therapy.
When people talk about bathing, they most often mean being immersed in water. But people have also "bathed" in other substances. Some therapies involve bathing in mud. People have also bathed in other substances, like Champagne, beans, or chocolate. A form of bathing quite common is bathing in sunlight.