Everybody uses this word but just few know what hatha yoga really means.
Hatha = willpower, but it refers to disciplined practice and behaviour using willpower and energy.
Sometimes Hatha Yoga is also translated as the "forcefull yoga", because it requires a physical effort. Hatha Yoga is certainly the yoga that is the best known in the West, which is part of the reason why so many definitions of Hatha Yoga exist.
In the West, Asanas and Pranayamas are generally assigned to Hatha Yoga - they are actually a part of Raja Yoga.
Hatha Yoga is the path of practice and purification - internal, as well as external. It is therefore an additional Yoga path to the four traditional paths. Regular practice of these exercises requires willpower and discipline and that is why this path is called Hatha Yoga.
HA-THA, also means sun and moon. This refers to the union and harmony of the sun and moon principles, which result from the practice of Hatha Yoga.