Tai chi (taiji), short for T’ai chi ch’üan, or Tai ji quan (tàijíquán; 太极拳), is an internal Chinese martial art practiced for both its defense training and its health benefits. The term taiji refers to a philosophy of the forces of yin and yang, related to the moves. Though originally conceived as a martial art, it is typically practiced for a variety of other personal reasons: demonstration competitions and achieving greater longevity, balance and to reduce knee pain. As a result, a multitude of training forms exist, both traditional and modern, with differing emphasis. Some training forms of tàijí quán are especially known for being practiced with relatively slow movements.