see below for other meanings
The philosophy reflected in the Hindu epics is the doctrine of the avatar (incarnation of a god as a human being). The two main avatars of Vishnu that appear in the epics are Rama, the hero of the Ramayana, and Krishna, the friend of the Pandavas in the Mahabharata. Unlike the superhuman devas (gods) of the Vedic Samhitas and the abstract Upanishadic concept of the all-pervading and formless Brahman, the avatars in these epics are the human intermediaries between the Supreme Being and mere mortals.
This doctrine has had a great impact on Hindu religious life, for to many it means that God has manifested Himself in a form that could be appreciated even by the least sophisticated. Rama and Krishna have remained prominent as beloved and adored manifestations of the Divine for thousands of years among Hindus. The Upanishadic concept of the all-embracing Brahman is revered by many to be the pinnacle of Indian thought, but the concept of the avatars has arguably had more influence on the average Hindu.
Rama and Krishna are by no means the only divine avatars in Hindu traditions. Hinduism includes the belief that the divine has taken human (and prior to the emergence of humankind, animal) forms here on earth many times. Little is known of any appearance as an avatar by Brahma, but emanations of the other two aspects of the Trimurti, Shiva and Vishnu, appeared a number of times. Even Gautama Buddha, founder of Buddhism, is regarded by many Hindus as an avatar of Vishnu.
The Maha Avatara (Great Avatars) of Vishnu are usually said to be ten (though a some Hindu scriptures list as many as 23):
Usage (by analogy with the above) in hacker culture:
1. Among people working on virtual reality and cyberspace interfaces, an avatar is an icon or representation of a user in a shared virtual reality. The term is sometimes used on MUDs, and also in computer role-playing games.
2. [CMU, Tektronix] root, superuser. There are quite a few Unix machines on which the name of the superuser account is 'avatar' rather than 'root'. This quirk was originated by a CMU hacker who found the terms 'root' and 'superuser' unimaginative, and thought 'avatar' might better impress people with the responsibility they were accepting.
Source: the public domain Jargon file