Definitions of Yoga from Source Texts (Credit Seth Powell)
The following 10 definitions of Yoga, from source texts are quoted from Seth Powell's course on the history of yoga which can be found here amongst others.
1. tāṃ yogam iti manyate sthirām indriyadhāraṇām
When the senses are firmly reigned in, that is thought to be yoga
— Kaṭha Upaniṣad 6.11
2. yogaś citta-vṛtti-nirodhaḥ
Yoga is the stilling of the fluctuations of the mind
— Yoga sūtra 1.2
3. ātmeśvara-saṃyogo yogaḥ
Yoga is the union of the self and the Lord
— Pañcārthabhāṣya 1.1.43 (comm. on Pāśupatasūtra 1.1)
4. samatvaṃ yoga ucyate
Equanimity is said to be yoga.
— Bhagavadgītā 2.48
5. yogaḥ karmasu kauśalam
Yoga is skill in action
— Bhagavadgītā 2.50
6. atha tattva-darśanābhyupāyo yogaḥ
Yoga is the direct means to perceive reality
— Unknown sūtra quoted in Śaṅkarācārya’s Brahmasūtrabhāṣya 2.1.3
7. ...ātmasthe manasi saśarīrasya sukha-duḥkhābhāvaḥ sa yogaḥ
When the mind is established in the Self, there is the absence of pleasure and pain for an embodied one. That is yoga.
— Vaiśeṣikasūtra 5.2.17
8. yogam ekatvam icchanti vastuno ’nyena vastunā
Yoga is said to be the oneness of one entity with another.
— Mālinīvijayottaratantra 4.4
9. Now I will teach yoga, together with its ancillaries, which bestows [pure] consciousness. Yogic experts state that yoga is the oneness of the individual soul (jīva) with the ātman. Others understand it to be the ascertainment of Śiva and the soul as non-different. The scholars of the Āgamas say that it is a Knowledge which is of the nature of Śiva’s Power. Other scholars say it is the knowledge of the primordial soul.
— Śāradātilaka 25.1-3b (trans. S. Vasudeva 2004)
10. The Goddess said: “What is yoga defined as, Lord, and what is achieved by yoga, tell me all of this, Śaṅkara!”
God said: “Yoga is defined as the unification of the many pairs of opposites, [such as] the unification of inbreath and outbreath, as also one’s blood and ones semen, the unification of the sun and the moon, [or] of the individual soul with the supreme soul.
— Yogabīja 88-90ab (trans. S. Vasudeva 2004)