Hatha yoga (Sanskrit हठयोग haṭhayoga, IPA: [ɦəʈʰəˈjoːɡə]), also called hatha vidya (हठविद्या), is a system of yoga introduced by Yogi Swatmarama, a sage of 15th century India, and compiler of the Hatha Yoga Pradipika. In this treatise Swatmarama introduces hatha yoga as preparatory stage of physical purification that the body practices for higher meditation. The asanas and pranayama in raja yoga were what the Hindu yogis used to physically train their body for long periods of meditation. This practice is called shatkarma.
The word hatha is a compound of the words ha and tha meaning sun and moon, referring to prana and apana, and also to the principal nadis (energy channels) of the subtle body that must be fully operational to attain a state of dhyana or samādhi. According to the Monier-Williams Sanskrit Dictionary, the word “hatha” means forceful. It is a strong practice done for purification. In other respects hatha yoga follows the same principles as the raja yoga of Patanjali. Hatha yoga is what most people in the Western world associate with the word “yoga” and is most commonly practiced for mental and physical health.
Definition courtesy Wikipedia.