Does Rig Veda contain the word yoga or its synonyms explicitly? If not, then which among the four Vedas explicitly contains the word Yoga?
My friend Sri Kiron Krishnan wrote on this topic in another site.
The word “yoga” as such appears in 6 mentions in the Rigveda. However it is only in three of them : 1.18.7, 1.30.7, 10.114.9 that the word assumes a meaning that is not literally related to the “yoking of horse”.
Yoga - Historical implications in Rigveda
The word yoga is derived from ‘yuj-’ meaning to “yoke”. It was initially used to mean the “yoking of horses” in Rigveda, which stood for the poetic implication of yoking the Self in the rays of spiritual dawn. (usually called the “bay/tawny horses” - hari-ashva in Rigveda) Thus, yoga is the art of yoking yourself with the spiritual rays of the Dawn, the Uṣas, which is facilitated by Indra or Brahmaṇaspati. The Indra, through his bay horses yoked to his vacoyuja (yoked with the word; word of the poet/devotee) viśvasammiśla (universally mingling) chariot, gets to the mind of devotee to help him in his spiritual struggle against the Vr̥tras inside.
Thus, in the inner world, the yoga is a spiritual activity, which promotes the spiritual journey.
The mentions and meanings in Rigveda
As you see, yoga deviates from even its literal meaning of yoking horses to the derived metaphorical meaning inside the Rigveda itself. In 1.18.7, the mention is :
sa dhīnāṁ yogaṁ invati
“He (saH) promotes (invati) the yoga (yogam) of thoughts (dhInAm)”. (He here is again, Brahmaṇaspati) In this mention, it is clear that Rigveda has itself showed to us what the “bay horses yoked by Indra / Brahmanaspati” are, in the more lucid part of first Mandala. Thus, it is clear from the mention that yoga is purely a spiritual activity, it has less got to do with physical exercises in Rigveda.
The second important mention is at 1.30.7 :
yoge yoge tavastaraṁ vāje vāje havāmahe
“In each yoga, we invoke the Strong (Indra); in each struggle”.
The third relevant mention is a part of a very mystic but beautiful hymn of Rigveda, in 10.114.9 :
“kaś chandasāṁ yogaṁ ā veda dhīraḥ ko dhiṣṇyāṁ prati vācaṁ papāda kam r̥tvijāṁ aṣṭamaṁ śūraṁ āhur harī indrasya ni cikāya kaḥ svit”
“who knows the yoga of the metres here, who has gained the “word” (Vak) the subject and object of thoughts? who is called the eighth Hero among the conductors of order? who has perhaps controlled the (two) bay horses of Indra!”
My regular readers may not find problem in understanding the various meanings “who” can have in a Rigvedic poem. Here, it is simply asked as a question, and this hymn follows that it is indeed the One “who” who does all this. Anyway, note the connection of the “yoga” of metres and the speech. It refers to the spiritual process by which the sage composes the poems, the vAk.
Thus, from these mentions of yoga, it should be clear that yoga in Rigveda means the spiritual yoking, the synchronizing of the divine thoughts / speech with the spiritual car of mind to start the spiritual journey. Its all poetic, don’t blame the poetic seers. Blame it on your curiosity. Blame it on my inefficiency.
The basic idea of Yoga is yoking by thought or prayer. Rig Veda does talk about yoking by thought or prayer.
The might which Usana hath formed for thee with might rends in its greatness and with strength both worlds apart. O Hero-souled, the steeds of Vata, yoked by thought, have carried thee to fame while thou art filled with power.
Rig Veda I.51.10
Slayer of Vrtra, mount thy car, thy Bay Steeds have been yoked by prayer. May, with its voice, the pressing stone draw thine attention hitherward.
Rig Veda I.84.3
There is one shloka where harnessing thought is mentioned.
The priests of him the lofty Priest well-skilled in hymns harness their spirit, yea, harness their holy thoughts...
There is a persistent effort in some circles to claim that Yoga came into Hinduism from Buddhism and Jainism. However the idea that something can be yoked by thought or prayer is there in the Rig Vedas.