Aum, the primordial sound, was present at the creation of the universe. It is said to be the original sound that contains all other sounds, all words, all languages and all mantras. The letter A represents creation, when all existence issued forth from Brahma's golden nucleus; the letter U refers to Vishnu the god of the middle who preserves this world by balancing Brahma on a lotus above himself, and the letter M symbolizes the final part of the cycle of existence, when Vishnu falls asleep and Shiva has to breathe in so that all existing things have to disintegrate and are reduced to their essence to him.

Answered by Gypcywoman , an ibibo Master, at 12:28 AM on September 21, 2007