Bhagavad Gita Chapter 6, Verse 23

तं विद्याद्दुःखसंयोगवियोगं योगसञ्ज्ञितम् | स निश्चयेन योक्तव्यो योगोऽनिर्विण्णचेतसा ||

taṁ vidyād duḥkha-saṁyoga-viyogaṁ yoga-saṁjñitam sa niścayena yoktavyo yogo 'nirviṇṇa-cetasā

Know that state called yoga, which is the disconnection from union with pain; it should be practiced with determination and an undespairing mind.

Krishna defines yoga and describes how to practice it. 'Taṁ vidyād duḥkha-saṁyoga-viyogaṁ yoga-saṁjñitam'—know (vidyāt) that (tam) state called (saṁjñitam) yoga (yoga), which is the disconnection (viyogam) from union (saṁyoga) with pain (duḥkha). This means: yoga is the state where you're disconnected from pain—you're no longer united with suffering, no longer identified with it, no longer bound by it. You're free from the connection to pain. 'Sa niścayena yoktavyaḥ'—that (saḥ) should be practiced (yoktavyaḥ) with determination (niścayena). This means: practice yoga with resolve, with commitment, with firmness—don't be half-hearted. 'Yogo 'nirviṇṇa-cetasā'—yoga (yogaḥ) with undespairing (anirviṇṇa) mind (cetasā). This means: practice with a mind that doesn't despair, that doesn't give up, that doesn't get discouraged. Even when progress is slow, even when challenges arise, don't despair—keep practicing. This is the definition of yoga—disconnection from pain—and the attitude for practice—determination and undespairing mind.