Bhagavad Gita · Chapter 18: Freedom through Renunciation

Bhagavad Gita 18.33

dhṛtyā yayā dhārayate manaḥprāṇendriyakriyāḥ | yogenāvyabhicāriṇyā dhṛtiḥ sā pārtha sāttvikī ||

The steadfastness by which, through unwavering yoga, a person holds firm the activities of the mind, the breath, and the senses: that steadfastness, Arjuna, is of clarity.

Modern context

Sattvic steadfastness holds the mind, breath and senses steady through unwavering yoga. It is not grim endurance but calm stability. The mind does not scatter, the breath does not race, the senses do not pull in all directions. This kind of firmness is what lets a person keep their balance in the middle of difficulty.