The word tapas comes from the root-verb ‘tap’ (santāpe, in the sense of suffering). Hence it means any discipline that causes suffering to the body-mind complex.
It is also used in the Upaniṣads like the Chāndogya (5.10.1, 2) and the Muṇḍaka (1.2.11) in the sense of performance of one’s duties and austerities.
Dharmasūtras like those of Gautama (19.15) equate it with a number of observances like brahmacarya (celibacy), satyavacana (speaking the truth), ārdra-vastratā (wearing wet clothes on the body until they dry up), adhaśśāyitā (sleeping on the bare ground), ahiṁsā (non-injury) and astainya (not depriving anyone of his possessions) and so on. The duration for the observance of these as expiations varies from a year up to one day, depending on the nature of sin.
The Bhagavadgītā (17.14-19) deals with tapas from three angles: śārīra (physical), vāṅmaya (as pertaining to speech) and mānasa (mental). Honouring and serving gods and holy persons, cleanli-ness, celibacy and non-violence belong to the first category. Sweet but truthful speech belongs to the second category. Control of mind and purity of emotions are the essential elements of the third.
The Gītā further qualifies these three kinds from the standpoint of the three guṇas. Sāttvik tapas is that which is performed by persons of pure character possessing concentration and faith. The tapas performed by persons who are hypocrites, for selfish benefits like name and honour, is termed rājasik. The one undertaken by foolish persons to harm others is classed at tāmasik.