A pool, wading pool, or swimming pool is a structure designed to hold water to allow swimming or alternative leisure activities. Pools can be built to the floor (in-ground pools) or constructed above ground (as a freestanding structure or as an element of architecture or other more substantial construction ), and are also a frequent feature aboard ocean-liners and cruise ships. In-ground pools are most frequently constructed from materials such as concrete, natural stone, metal, plastic or fiberglass, and can be of custom size and shape or constructed to standardized dimensions, the biggest of which is the Olympic-size pool.
Many health clubs, fitness centers and private clubs, such as the YMCA, have pools used mostly for recreation or exercise. Many cities and towns give public pools. Many resorts have pools available for their guests to use at their leisure. Educational facilities such as universities typically have pools for physical education courses, recreational activities, entertainment or competitive athletics like swimming teams. Hot tubs and spas are pools filled with hot water, used for comfort or hydrotherapy, and also are common in homes, hotels, and health clubs. Unique swimming pools are also used for diving, technical water sports, physical treatment in addition to for the practice of lifeguards and astronauts. Swimming pools may be heated or unheated.