Yoga is not a modern invention of the human mind but our ancient and prehistoric possession. The Veda is our oldest extant human document and the Veda, from one point of view, is a great compilation of practical hints about Yoga. All religion is a flower of which Yoga is the root; all philosophy, poetry and the works of genius use it, consciously or unconsciously, as an instrument. We believe that God created the world by Yoga and by Yoga He will draw it into Himself again. Yogah prabhavapyayau, Yoga is the birth and passing away of things. When Sri Krishna reveals to Arjuna the greatness of His creation and the manner in which He has built it out of His being by a reconciliation of logical opposites, he says, “Pasya me yogam aishwaram.” Behold my divine Yoga. We usually attach a more limited sense to the word; when we use or hear it, we think of the details of Patanjali’s system, of rhythmic breathing, of peculiar ways of sitting, of concentration of mind, of the trance of the adept. But these are merely details of particular systems. The systems are not the thing itself, any more than the water of an irrigation canal is the river Ganges. Yoga may be done without the least thought for the breathing, in any posture or no posture, without any insistence on concentration, in the full waking condition, while walking, working, eating, drinking, talking with others, in any occupation, in sleep, in dream, in states of unconsciousness, semiconsciousness, double-conciousness. It is no nostrum or system or fixed practice, but an eternal fact of process based on the very nature of the Universe.
Nevertheless, in practice the name may be limited to certain applications of this general process for specific and definite ends. Yoga stands essentially on the fact that in this world we are everywhere one, yet divided; one yet divided in our being, one with yet divided from our fellow creatures of all kinds, one with yet divided from the infinite existence which we call God, Nature or Brahman. Yoga, generally, is the power which the soul in one body has of entering into effective relation with other souls, with parts of itself which are behind the waking consciousness, with forces of Nature and objects in Nature, with the Supreme Intelligence, Power and Bliss which governs the world either for the sake of that union in itself or for the purpose of increasing or modifying our manifest being, knowledge, faculty, force or delight. Any system which organises our inner being and our outer frame for these ends may be called a system of Yoga.
Excerpt from the book Essays Divine and Human by Sri Aurobindo