Om (also spelled Aum) is a Hindu sacred sound that is considered the greatest of all mantras.

The syllable Om is composed of the three sounds a-u-m (in Sanskrit, the vowels a and u combine to become o) and the symbol’s threefold nature is central to its meaning. It represent several important triads:
  • the three worlds —earth, atmosphere, and heaven
  • the three major Hindu gods —Brahma, Vishnu, and Siva
  • the three sacred Vedic scriptures—Rg, Yajur, and Sama

Thus Om mystically embodies the essence of the entire universe. This meaning is further deepened by the Indian philosophical belief that God first created sound and the universe arose from it. As the most sacred sound, Om is the root of the universe and everything that exists and it continues to hold everything together.

Om is spoken at the beginning and the end of Hindu mantras, prayers, and meditations and is frequently used in Buddhist and Jain rituals as well. Om is used in the practice of Yoga and is related to techniques of auditory meditation. From the 6th century, the written symbol of Om was used to mark the beginning of a text in a manuscript or an inscription. Om Parvat, a sacred peak at 6191m in the Indian Himalayas, is revered for its snow deposition pattern that resembles Om.

Source: Nitin Kumar and Encyclopædia Britannica