The term avatar is derived from ava + tri which means descent or act of descending. Dr S Radhakrishnan explains thus – Avatara means descent, one who has descended. The divine comes down to the earthly plane to raise it to a higher status. God descends when man rises. The purpose of the avatara is to inaugurate a new world, a new dharma. By teaching and example, he shows how a human being can raise himself to higher grade of life. The issue between the right and wrong is a decisive one. God words on the side of the right. Love and mercy are ultimately more powerful than hatred and cruelty. Dharma will conquer adharma, truth will conquer falsehood; the power behind death, disease and sin will be overthrown by the reality which is Being, Intelligence and Bliss. (Bhagavad Gita – S Radhakrishnan page 155)
Sri Aurobindo, however, emphasizes that among the functions of an avatar the upholding of dharma is not an all-sufficient object in itself, not the supreme possible aim for the manifestation but is only the general condition of a higher aim and a more supreme and divine utility.
For there are two aspects of the divine birth; one is descent, the birth of God in humanity, the Godhead manifesting itself in the human form and nature of the eternal avatar; the other is ascent, the birth of man into the Godhead, man rising into the divine nature and consciousness, madbhavan agatah; it is the being born anew in a second birth of the soul.
The theory of the avatars appears in a full-fledged form for the first time perhaps in the Bhagavad Gita (IV, 6, 7, 8) where Bhagavan Sri Krishna says to Arjuna – Though I am unborn, and My self is imperishable, though I am the God of all creatures, yet establishing myself in my own nature, I come into empiric being through my power (maya). Whenever there is a decline of righteousness and rise of unrighteousness, O Bharata (Arjuna), then I send forth or create or incarnate myself. For the protection of the good, for the destruction of the wicked and for the establishment of righteousness, I come into being from age to age.