Om is a sacred sound and a sacred spiritual symbol in Hinduism, that signifies the essence of the ultimate reality, consciousness or Soul. Some believe it is the sound or vibration of creation.
The syllable Om is first mentioned in the Upanishads, the mystical texts associated with the Vedanta philosophy. It has variously been associated with concepts of "cosmic sound" or "mystical syllable" or "affirmation to something divine", or as symbolism for abstract spiritual concepts in the Upanishads.In the Aranyaka and the Brahmana layers of Vedic texts, the syllable is so widespread and linked to knowledge, that it stands for the "whole of Veda". The etymological foundations of Om are repeatedly discussed in the oldest layers of the Vedantic texts (the early Upanishads). The Aitareya Brahmana of Rig Veda, in section 5.32, for example suggests that the three phonetic components of Om (pronounced AUM) correspond to the three stages of cosmic creation, and when it is read or said, it celebrates the creative powers of the universe. The Brahmana layer of Vedic texts equate Om with Bhur-bhuvah-Svah, the latter symbolizing "the whole Veda". They offer various shades of meaning to Om, such as it being "the universe beyond the sun", or that which is "mysterious and inexhaustible", or "the infinite language, the infinite knowledge", or "essence of breath, life, everything that exists", or that "with which one is liberated". The Sama Veda, the poetical Veda, orthographically maps Om to the audible, the musical truths in its numerous variations (Oum, Aum, Ovā Ovā Ovā Um, etc.) and then attempts to extract musical meters from it.
The syllable Om evolves to mean many abstract ideas in the earliest Upanishads. Max Müller and other scholars state that these philosophical texts recommend Om as a "tool for meditation", explain various meanings that the syllable may be in the mind of one meditating, ranging from "artificial and senseless" to "highest concepts such as the cause of the Universe, essence of life, Brahman, Atman, and Self-knowledge".