Who is an Avatar?

An ‘Avatar’ is defined as an incarnation (physical manifestation) of the Supreme Being. The word ‘Avatar’ is derived from the Sanskrit word ‘Avataranam’ which means ‘descent’, and usually implies a deliberate descent of the Divine into the mortal realms to reveal the Absolute Truth to humanity and remind them of their true divine nature. Though Avatars may appear in different forms at different times, places and circumstances, yet they are all the manifestations of the One Supreme Lord.
Bhagawan Sri Sathya Sai Baba, the Avatar of the Age, has succinctly explained the meaning and the reason for the descent of the Avatar in the following Telugu verse:

“Avatarinchuta yanutalo Arthamemi?Janulapai Preethi Vaatsalya Paratha thodaVaari Sthaayaiki Daivambu Vachchu BhuvikiJeeva Prajnatho baatuga Daiva prajna”

He says that Formless God takes a form and descends upon the Earth as an expression of His boundless love and affection towards humanity. The Avatar is an enigmatic, yet delightful blend of individual consciousness and the Divine Consciousness. The Avatar behaves in a human way so that mankind can feel kinship with Him, but rises to His super-human heights so that mankind can aspire to reach those heights.
In Bhagavad-Gita, Lord Sri Krishna, the Avatar of the Dwapara Age, declares thus:

"Yada Yada Hi Dharmasya Glanirbhavathi Bharatha Abhyukthanam adharmasya Thadathmanam Srujamyaham.”“Parithranaya Sadhunam Vinashaya cha Dhushkrutham Dharma samsthapanarthaya Sambhavami Yuge Yuge"

Whenever there is a decline of Righteousness and rise of evil, the Lord incarnates from time to time to uphold Righteousness, to protect the Virtuous and to uproot the evil.
The Avatar appears whenever the world is passing through a spiritual and moral crisis. The Avatar comes in order to uphold Dharma (Righteousness) and raise the universal consciousness.