The Sanskrit word avatāraḥ is derived from ava meaning ‘down’ or descent andtarati meaning ‘crossing over or tiding over’ and refers to the deliberate descent of a deity, immortal being or the Supreme Being from heaven (normally Viṣṇu) for specific purpose(s). As a noun, avatar also means a new personification of a familiar, idea1 like an embodiment2 or a temporary manifestation3.
God is one in Hinduism, to which there is no doubt and is represented by the sacred syllable om (ॐ).
1 WordNet, Princeton University
2 An embodiment, as of a quality or concept; an archetype: ‘the very avatar of cunning’. The American Heritage® Dictionary of the English Language, Fourth Edition; Dictionary.com
3 A temporary manifestation or aspect of a continuing entity: ‘occultism in its present avatar’. The American Heritage® Dictionary of the English Language, Fourth Edition; Dictionary.com