Winter swimming is the activity of swimming during the winter season, typically in outdoor locations or in unheated pools or lidos. In colder countries, it may be synonymous with ice swimming, when the water is frozen over. This requires either breaking the ice or entering where a spring prevents the formation of ice. It may also be simulated by a pool of water at 0 °C (32 °F), the temperature at which water freezes.

Swimming is the self-propulsion of a person through water, or other liquid, usually for recreation, sport, exercise, or survival. Locomotion is achieved through coordinated movement of the limbs and the body to achieve hydrodynamic thrust that results in directional motion. Humans can hold their breath underwater and undertake rudimentary locomotive swimming within weeks of birth, as a survival response. Swimming requires stamina, skills, and proper technique.