Tapas
तपस् (tapas)
Heat - the friction of voluntary discipline, by which effort burns off what mere intention cannot.
Tapas means heat, and the metaphor is metallurgical: ore becomes metal in fire. In practice it names voluntary difficulty - the fast kept, the cold accepted, the hour defended - undertaken not for punishment but for refinement.
The traditions are wary of tapas that performs. Heat that feeds pride merely hardens a different impurity, and the Gita explicitly sorts austerity into luminous, agitated, and deluded kinds. The test is what the discipline does to the rest of one's conduct: real tapas leaves a person softer with others and firmer with themselves, not the reverse.
Modern life offers abundant raw material - the phone left in another room, the difficult conversation not postponed, the early alarm honoured. Friction is not the obstacle to practice; chosen well, it is the practice.