Bhagavad Gītā Chapter Six verse 46

tapasvibhyo ’dhiko yogī jñānibhyo ’pi mato ’dhikaḥ |
karmibhyaś cādhiko yogī tasmād yogī bhavārjuna ||

tapas - to heat; religious austerity; self-discipline; pain, suffering

Commentaries and Reflections

Commentary by T Krishnamacharya:

Tapas is Proper Diet, Mantra Chanting and Self-Inquiry.