In Vedic Sanskrit, yoga (from the root yuj) means "to add", "to join", "to unite", or "to attach" in its most common literal sense. By figurative extension from the yoking or harnessing of oxen or horses, the word took on broader meanings such as "employment, use, application, performance" (compare the figurative uses of "to harness" as in "to put something to some use"). All further developments of the sense of this word are post-Vedic. More prosaic moods such as "exertion", "endeavour", "zeal", and "diligence" are also found in Indian epic poetry.
There are very many compound words containing yoga in Sanskrit. Yoga can take on meanings such as "connection", "contact", "union", "method", "application", "addition" and "performance". In simpler words, Yoga also means "combined". For example, guṇáyoga means "contact with a cord"; chakráyoga has a medical sense of "applying a splint or similar instrument by means of pulleys (in case of dislocation of the thigh)"; chandráyoga has the astronomical sense of "conjunction of the moon with a constellation"; puṃyoga is a grammatical term expressing "connection or relation with a man", etc. Thus, bhaktiyoga means "devoted attachment" in the monotheistic Bhakti movement. The term kriyāyoga has a grammatical sense, meaning "connection with a verb". But the same compound is also given a technical meaning in the Yoga Sutras (2.1), designating the "practical" aspects of the philosophy, i.e. the "union with the supreme" due to performance of duties in everyday life.
According to Pāṇini, a 6th-century BCE Sanskrit grammarian, the term yoga can be derived from either of two roots, yujir yoga (to yoke) or yuj samādhau (to concentrate). In the context of the Yoga Sutras of Patanjali, the root yuj samādhau (to concentrate) is considered by traditional commentators as the correct etymology. In accordance with Pāṇini, Vyasa who wrote the first commentary on the Yoga Sutras, states that yoga means samādhi (concentration).
According to Dasgupta, the term yoga can be derived from either of two roots, yujir yoga (to yoke) or yuj samādhau (to concentrate). Someone who practices yoga or follows the yoga philosophy with a high level of commitment is called a yogi (may be applied to a man or a woman) or yogini (traditionally denoting a woman)
United National Declaration for International Day of Yoga: