Hatha yoga (Sanskrit हठयोग hʌʈʰʌjo:gʌ), also calle Hatha Vidya (हठविद्या), is a particular 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 'a stairway to the heights of Raja Yoga', hence a preparatory stage of physical purification that renders the body fit for the practice of 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 practise is called shatkarma. The word Hatha is a compound of the words Ha and Tha meaning sun and moon ( हकारः कीर्तितः सूर्यष्ठकारश्चंद्र उच्यते | सूर्यचंद्रमसोर्योगाद्धठयोग निगद्यते || ), referring to Praana and Apaana, and also to the principal nadis (energy channels) of the subtle body that must be fully operational to attain a state of dhyana or samadhi. According to the Monier Moneir-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 Raaja Yoga of Patanjali including moral restraint yama and spiritual observances niyama. Hatha Yoga is what most people in the West associate with the word "Yoga" and is practiced for mental and physical health throughout the West.