The word Yoga seems to have acquired such a prolific significance all along the history and development of its various practices, even from the earliest period of the Vedas, that it is very difficult to associate or determine its exact meaning within the limits of any particular philosophy. For example, during the period of the Rigveda and in such other later Vedic works as the Brahmanas and the Aranyakas, the word Yoga meant "achieving the unachieved," "harnessing," "controlling," "yoking," "connection," and the like. The word has also another derivative, "Yogya," which is probably an old word of the Aryan stock used in later Sanskrit literature. Again, Yoga also means effort (Kriya) in the early literature of the Upanishads. This probably is due to the use of the word Yuj, particularly in the Atmanepada, which came to mean to prepare for work or to exert oneself. Similar to the old metaphor of controlling or yoking the senses, which were considered as uncontrollable horses, Yoga as an effort was probably suggested by a metaphor taken from the stretching of a bow.
During the period of the earlier Upanishads, however, the word Yoga passed through a variety of meanings and interpretations. In the Katha Upanishad (VI, 11), we are told that Yoga is the firm restraint of the senses, i.e., when the senses are controlled with the mind and when the mind becomes steady — that controlling and keeping steady of the senses and the mind is Yoga. This was followed by an important addition and modification of the original idea, viz., the control of the Prana as representing the process of Yoga. Again, the idea of absorption and union with the Supreme Being, which occurs in the Upanishads, especially in the Taittiriya (X), Mundaka (II, 4), and Katha (IV, 15), seems also to have greatly influenced the later Yoga interpretations. In the Maitrayaniya Upanishad, however, Yoga is defined as the joining of the Prana and Om, and also as the uniting of the senses, the mind, and the Prana by the removal of all sensual knowledge (Bhava). It also holds that Yoga is one-pointedness and unity (yogam lakṣy ekatvam iti). In the commentary called Dipika of Ramatirtha on the same Upanishad, we are told that relative to the control of the mind-functions, the Yoga proposes the following, viz., the six accessories (cittaikāgrya-sādhanasya prayoga-vidhi). We further read that the ultimate aim of all is to go back to the Infinite (brahmatma-tattvam sā gatiḥ puruṣārthaḥ). Then the mind, having thus absorbed (vilīne) itself with all that belongs to it (manśe means manaḥ-pariṇāma-rūpe Śaṅkara granthai), shines forth in its own nature of consciousness (diptimat jñāna-svabhāvam). In the Mahabharata, Yoga is defined as unity with the source (in the sense of Laya Yoga). At another place, it advocates that the Yogins are those who are well connected (with the object of their knowledge). It also refers to the concentration of the mind as Yoga (195, 528). In an important early work of the Pancaratra Agama, Yoga is defined as the union of the soul (Jiva) with the Oversoul (Paramatma). This ideal of unity and absorption seems to have become quite popular during the period of the Puranas. Thus, in the Vishnu Purana (VI, 31), we read: "that particular mental attempt which absorbs itself in Brahman is known as Yoga." From the elaboration of Yoga as given in the Brahmapurana (235), however, we understand that the union of the senses and the mind is also Yoga. There are a number of misinterpretations of Yoga in the various Puranas, e.g., in the Aditya Purana, in the Skanda Purana, and in the Linga Purana. Besides these, there are over a hundred definitions of Yoga in the later Yoga Upanishads and other works. In the Bhagavad Gita, apart from such definitions as the equability of mind (II, 48) and dexterity in practical life (II, 50) as Yoga, which are no doubt original interpretations, a novel use of the word Brahmabhuta (Brahman-absorbed) relative to the practice of Yoga is extremely suggestive. Thus, we are told that the Yogin becomes Brahmabhuta or Brahmamaya. In this state, the Self is not perceived, for it is only when the same has gone out of entire existence and not even the slightest trace of it is left that it is called Brahmabhuta. This idea does not seem to be so familiar to the Upanishad sages. The ultimate reality in the Upanishads, as merging of the salt in water, should be treated as only theoretical, for they do not talk of the process by which the same can be successfully achieved. It is only Yoga (parama-puruṣārtha-sādhana) which leads to this final stage.
Even from the earliest period, the achievement of anything unusual was credited to the knowledge of Yoga. It thus came to be associated with miraculous powers, which both Hinduism and Buddhism have made their own. It has therefore been admitted that there is no power greater than Yoga (nāsti yoga-samam balam) and that by the power of Samadhi, the Yogin can expect to achieve anything and everything. As a further development, even the results of Yoga came to be known as Yoga. For example, in Raja Tarangini, entering into another body is called Yoga. We are further told by Charaka that through Yoga, even a powerful poison can become good medicine.
During Panini's time, however, the word Yoga seemed to have attained its technical meaning, and a distinction was made between Yoga in the sense of concentration (from the root yuj samādhau) and connection or union (from the root yujir yoge). Patanjali accepted the first meaning of concentration and defined Yoga as the restraining of the various mental modifications (yogaś citta-vṛtti-nirodhaḥ). That particular state of the mind-stuff (Citta), therefore, in which the modifications (Vritti) of real cognition, misconception, imagination, sleep, and memory (as manifestations of activity or Rajas and inertia or Tamas nature of the Citta) have been restrained, is the state of Yoga. As its achievement, Patanjali mentions the knowledge of the difference between Purusha and Prakriti, which ultimately leads to aloneness (Kaivalya).