Even though Om is such a short word, its meaning is huge. Om (also spelled Aum) is a Hindu sacred sound that is considered the greatest of all mantras. I found an explanation of Om in the fabulous book Heart of Yoga by TKV Desikachar.

When we analyze the meaning of Om the way it is written in Sanskrit, it is made up of A, U, M and a symbol representing resonance. So Om has four aspects. The first one is A, a sound that comes from the belly, is formed in the open throat and is voiced with the mouth open. As with many alphabets, A is the first letter of the Sanskrit alphabet. The second sound is the U, a sound that is formed in the middle of the mouth. The third sound is M, with this the mouth closes. M is the final consonant in Sanskrit alphabet. Then the sound rises to the nasal passage from where the resonance is made.

Om, Aum gets through from A to M through U. This represents everything that can be expressed in letters and words. When we say A, we open the mouth, which means the process of creation. The sound U symbolizes the continuance of creation, which is constantly renewing itself. M symbolizes the end and dissolution. Following this, the sound of Om continues for a while. This sound has no alphabetical symbol. This means that Om contains everything that can be expressed in words and which cannot be expressed in words. This is the full meaning of Om.

The Hindu Upanishads say that A represents the waking state, U the dream state, M the state of deep dreamless sleep, the fourth sound following the M is the samadhi. This parallel points to the One who stands behind all four states, the only one who is truly awake, Ishvara. There is One who is present in all these states, One who never sleeps, never dreams, One is who is always watchful, One who knows about everything and yet is beyond everything. By repeating Om, one can gradually become immersed in Ishvara, the mind becomes saturated and still.

By saying Om one is calling on all the following–

Brahma for creation, Vishnu for sustenance and Shiva for destruction

Physical or earth plane, mental or atmospheric plane, astral or cosmic plane

Past, present, future

Birth, life, death

The sanitized non Hindu version of meaning Om is this- Om is the whole universe coalesced into a single sound and represents the union of mind, body, and spirit that is at the heart of yoga.

Aaaaauuuuummmm.