A swimming pool, swimming bath, wading pool, paddling pool, or simply pool is a structure designed to hold water to enable swimming or other leisure activities. Pools can be built into the ground (in-ground pools) or built above ground (as a freestanding construction or as part of a building or other larger structure), and are also a common feature aboard ocean-liners and cruise ships. In-ground pools are most commonly constructed from materials such as concrete, natural stone, metal, plastic, or fiberglass, and can be of a custom size and shape or built to a standardized size, the largest of which is the Olympic-size swimming pool.
Many health clubs, fitness centers, and private clubs have pools used mostly for exercise or recreation. Many towns and cities provide public pools. Many hotels have pools available for their guests to use at their own leisure. Educational facilities such as high schools and universities sometimes have pools for physical education classes, recreational activities, leisure, and competitive athletics such as swimming teams. Hot tubs and spas are pools filled with hot water, used for relaxation or hydrotherapy. They are common in hotels and health clubs, and can also be found in many private homes. Special swimming pools are also used for diving, specialized water sports, and physical therapy, as well as for the training of lifeguards and astronauts. Swimming pools may be heated or unheated.