The syllable Aum is first described as all-encompassing mystical entity in the Upanishads. Today, in all Hindu art and all over India and Nepal, 'Aum' can be seen virtually everywhere, a common sign for Hinduism and its philosophy and mythology
With preceding a or ā, the o of om in Sanskrit grammar in sandhi (Sanskrit: संधि, "joining") does not form vriddhi (au) but guna (o) per Pāini 6.1.95.
The Sanskrit name for the syllable is pranava, from a root nu "to shout, sound, praise", verbal pra-nu- being attested as "to make a humming or droning sound" in the Brahmanas, and taking the specific meaning of "to utter the syllable om" in the Chandogya Upanishad and the Shrauta Sutras. More rarely used terms are akara or ekākara, and in later times omkāra becomes prevalent.
A popular depiction of the Aum syllable in the Devanagari script () is a ligature of ओ+ँ (oṃ, encoded in Unicode at U+0950 ॐ, the Tibetan script variant ༀ at U+0F00, and the Chinese 唵 at U+5535 or 吽 at U+543D).
It is also believed that after a very long time of meditation the Purusha Sukta revealed the word AUM as being the truth
In Puranic Hinduism, Aum is the mystic name for the Hindu Trimurti, and represents the union of the three gods, viz. a for Brahma, u for Vishnu and m for Mahadev which is another name of Shiva. The three sounds also symbolise the three Vedas (Rigveda, Samaveda, Yajurveda).
According to Hindu philosophy(see Mandukya Upanishad), the letter A represents creation, when all existence issued forth from Brahma's golden nucleus; the letter U refers to Vishnu the god of the middle who preserves this world by balancing Brahma on a lotus above himself, and the letter M symbolizes the final part of the cycle of existence, when Vishnu falls asleep and Shiva has to breathe in so that all existing things have to disintegrate and are reduced to their essence to him. More broadly, Aum is said to be the primordial sound that was present at the creation of the universe. It is said to be the original sound that contains all other sounds, all words, all languages and all mantras.