(tapasya)
Tapas (heat) or tapasya is a concept of great importance in Hindu practices of austerity. The word refers to the sacred heat that is generated by bodily mortifications and ascetic denial. Tapas confers wondrous powers and abilities upon the YOGI and makes it easier for the adept to break the bonds of attachment to worldly life.
In a very literal example of tapas, ancient orthodox yogis used to sit surrounded by five fires under the noonday Sun in order to absorb the heat and gain spiritual power. Other ascetics doing tapasya would endure lengthy fasts or extreme bodily mortifications. There is some indication that the notion of tapasya developed within the Brahminical tradition as a result of association with the fires of the Vedic sacrifice.
Further reading: Walter O. Kaelber, Tapta Marga: Asceticism and Initiation in Vedic India (Albany: State University of New York Press, 1989); Klaus K. Klos-termaier, A Survey of Hinduism (Albany: State University of New York Press, 1990); David M. Knipe, In the Image of Fire: Vedic Experiences of Heat (Delhi: Motilal Banar-sidass, 1975).
Encyclopedia of Hinduism. A. Jones and James D. Ryan. 2007.