Om, also written Aum and rendered in Devanagari as a single ligature, is the most concentrated sacred sound of the Indian traditions. The Mandukya Upanishad, one of the shortest of the principal Upanishads, is devoted entirely to it, analysing the syllable into three morae, A, U and M, identified with the waking, dream and deep-sleep states, and a silent fourth (turiya) beyond them, identified with the absolute. The Chandogya Upanishad opens by calling the udgitha, the sung Om of the Samaveda liturgy, the essence of all essences. Om thus stands at the junction of ritual chant and metaphysics: at once a practical liturgical vocable that opens and closes Vedic recitation and mantra, and a symbol of ultimate reality (Brahman, identified with the Atman or self).
The Mandukya Upanishad's analysis of Om into three sounds and a silent fourth became one of the most commented-upon short texts in the Vedantic tradition, most influentially through Gaudapada's Karika, a verse commentary of early but uncertain date reading the syllable's structure as a map of consciousness, waking, dreaming and dreamless sleep understood as states the seeker passes through toward the silence beyond sound that is turiya, pure awareness. Because the Upanishad treats Om as identical with Brahman rather than merely symbolic of it, later Vedanta philosophers built entire systems on the claim that meditating on the syllable is direct engagement with ultimate reality, a stronger claim than most traditions make for their own sacred words. In Tibetan Buddhist mantra Om is paired with Ah and Hum to mark body, speech and mind at the head of longer formulas such as Om mani padme hum, keeping Om's Vedic role as an opening, consecrating sound while attaching it to an entirely different doctrinal system.