Om also Aum
The supreme and most sacred syllable, consisting in Sanskrit of the three sounds (a), (u), and (m), representing various fundamental triads and believed to be the spoken essence of the universe. It is uttered as a mantra and in affirmations and blessings.
OM
The imperishable sound, is the seed of all that exists.
The past, present, the future, all are but the unfolding of OM.
And whatever transcends the three realms of time, that indeed is the flowering of OM.
This pure self and OM are as one, and the different quarters of the self correspond to OM and its sounds. – Mandukya Upanishad