Bhagavad Gita · Chapter 2: The Path of Understanding
Bhagavad Gita 2.48
योगस्थः कुरु कर्माणि सङ्गं त्यक्त्वा धनञ्जय ।
सिद्ध्यसिद्ध्योः समो भूत्वा समत्वं योग उच्यते ॥
yogasthaḥ kuru karmāṇi saṅgaṃ tyaktvā dhanañjaya | siddhyasiddhyoḥ samo bhūtvā samatvaṃ yoga ucyate ||
Settled in yoga, Arjuna, do your work, giving up attachment and remaining the same in success and failure. This evenness of mind is called yoga.
Modern context
Here is the Gita's own definition of yoga: evenness of mind. Not a posture and not a retreat from the world, but remaining steady in the middle of work, whether it succeeds or fails. By this definition a person can practise yoga at a desk, in a kitchen or in a hospital ward.