तं विद्याद् दुःखसंयोगवियोगं योगसंज्ञितम्। स निश्चयेन योक्तव्यो योगोऽनिर्विण्णचेतसा ॥

taṁ vidyād duḥkha-sanyoga-viyogaṁ yogasaṅjñitam sa niśhchayena yoktavyo yogo ’nirviṇṇa-chetasā

That should be known by the name of yoga, this disconnection from the union with pain. This yoga must be clung to with a firm conviction and with undesponding mind.

In plain words: Krishna says yoga means being cut off from pain. A person should stick with this practice firmly and without losing heart.