tummo (Tibetan, gtum-mo; Skt., caṇḍalī). The energy channel that is thought to extend upwards from the heart centre in some forms of tantric practice. The term is also used derivatively for the yogic practice of the completion-phase which results in the generation of inner heat as practised by Milarepa.

A Dictionary of Buddhism, Oxford University Press, 2003, 2004 (which is available in electronic version from answer.com)