Most of us picture a pose. The Gītā defines yoga twice in three verses: evenness of mind, and skill in action. Neither is a pose.
The verses
BHAGAVAD GĪTĀ · 2.48योगस्थः कुरु कर्माणि सङ्गं त्यक्त्वा धनञ्जय।
सिद्ध्यसिद्ध्योः समो भूत्वा समत्वं योग उच्यते।।
yoga-sthaḥ kuru karmāṇi saṅgaṁ tyaktvā dhanañjaya
siddhy-asiddhyoḥ samo bhūtvā samatvaṁ yoga ucyate
Abandoning attachment and established in Yoga, perform works, viewing success and failure with an even mind. Evenness of mind is said to be Yoga.
BHAGAVAD GĪTĀ · 2.50बुद्धियुक्तो जहातीह उभे सुकृतदुष्कृते।
तस्माद्योगाय युज्यस्व योगः कर्मसु कौशलम्।।
buddhi-yukto jahātīha ubhe sukṛta-duṣkṛte
tasmād yogāya yujyasva yogaḥ karmasu kauśalam
A person with an evenness of mind discards both good and evil here and now. Therefore, strive for Yoga; for Yoga is skill in action.
Going deeper
Within a few verses the Gītā defines yoga twice, and neither definition is a pose. 2.48 says: abandon attachment, stand in yoga, do your work, and treat success and failure alike. Evenness of mind, the verse says, is yoga. 2.49 adds that action done with attachment is far lower than action done with evenness of mind, and that those who act for results are miserable. 2.50 gives the second definition: a person with an even mind casts off both good and evil deeds here and now, so strive for yoga, because yoga is skill in action. 2.51 shows what the wise do with that: they give up the fruits of action and are freed from the bond of birth. All of this sits in chapter 2, and what the Gītā says about karma follows it further.