Om is a word used by Hindu yogis to represent a vibration which they say pervades the entire universe. They believe this is the same sound as the one heard internally as a result of practicing yoga.*

More generally, Om represents God, the supreme, all that is.

Om is used as a mantra (a word or phrase that's said repeatedly out loud or in one's head) in Japa Yoga.

Aum is a variant spelling of Om. The word can be spelled either way because the letter o is regarded as a diphthong consisting of a and u.

According to one of the most famous Hindu scriptures, the Mandukya Upanishad, Om symbolizes the four states of consciousness. The letter a represents the waking state, u represents the dream state, m represents deep sleep, and the whole word represents the fourth state (turiya), which is the state of enlightenment.

Pranava is a name used for the syllable Om. For example, you might say, "I recited the pranava a thousand times," meaning you said "Om" a thousand times. Literally, pranava means "pronouncing" in Sanskrit. The word consists of the prefix pra (a cognate of the Latin prefix pro) and the root nu meaning "call out" and "exult."

Pranava also means the sound that people hear internally after they practice yoga for a while.

The place of these concepts in yoga is summarized nicely by this paragraph from the website of the Himalayan Academy:

Literally, Pranava in Sanskrit means "humming." The mantram [mantra] Aum denotes God as the Primal Sound. This sound can be heard as the sound of one's own nerve system, and meditators and mystics hear it daily, like the sound made by an electrical transformer or a swarm of bees, or a thousand vinas playing in the distance. It is a strong, inner experience, one that yogis hold with great reverence. The meditator is taught to inwardly transform this sound into the inner light which lights up ones' thoughts, and to bask in this blissful consciousness of light. Pranava is also known as the sound of the nadanadi sakti. Hearing it one draws near to God Consciousness. When we are living in the lower chakras, or when the world too strongly dominates our mind, this sound may, for a time, not be heard. But it returns as awareness withdraws, as the mind becomes perfectly quiescent, silent, still. Listen for this sound in your quietest moments and you will learn to recognize it as a daily encounter with the Divine that lives within all men, within all creatures, within all existence. Aum Namasivaya!

Some skeptics say that the humming heard by experienced meditators is actually a form of self-induced epilepsy.

Illustration copyright Sharon Webb.

Mandukya Upanishad
A classic Sanskrit scripture that explains the significance of Om in terms of the four states of consciousness..

Sanskrit
Our main reference page on the Sanskrit language.

The Sacred Syllable Om
by Georg Feuerstein
A superb article on the history and meaning of Om. On the website of the Yoga Research and Education Center.

Aum Homepage of the Himlayan Academy
Artwork, explanations, and quotations from famous teachers. This is a terrific site.

Commentary on the Mandukya Upanishad
by Swami Krishnananda

Atharva-Shikha-Upanishad
This classic Sanskrit scripture is all about Om.

Fractal Images By Sharon Webb
Beautiful digital art and music by artist/composer Sharon Webb.