Oṃ, also written Auṃ and called the praṇava, is chanted at the beginning and end of Vedic recitation and prayer. The Upaniṣads make it a symbol and support of meditation on Brahman; the Māṇḍūkya analyses its three sounds as the waking, dreaming and sleeping states, and the silence after it as the fourth. It is also sacred in Buddhism, Jainism and Sikhism.
Oṃ
The sacred syllable of the Hindu traditions, said to contain all sound and to stand for Brahman.
Also: Om, Aum, pranava