tapas, whose original meaning is ‘extreme heat’ but which came to designate ascetic effort in general. Tapas is definitely documented in the Rg-Veda, and its powers are creative on both the cosmic and the spiritual plane
Eliade establishes tapas as the pan-Indian concept of mystical heat whose creative power operates simultaneously on cosmogonic and individual spiritual planes, linking shamanic ecstasy to ascetic practice.
, Shamanism: Archaic Techniques of Ecstasy, 1951thesis