Om (or Aum; listen^ⓘ; Sanskrit: ॐ, ओम्, romanized: Oṃ, Auṃ, ISO 15919: Ōṁ) is a polysemous symbol representing a sacred sound, seed syllable, mantra, and invocation in Hinduism as well as in other Indian religions. Its written form is the most important symbol in the Hindu religion. It is the essence of the supreme Absolute, consciousness, Ātman, Brahman, or the cosmic world. In Indian religions, Om serves as a sonic representation of the divine, a standard of Vedic authority and a central aspect of soteriological doctrines and practices. It is the basic tool for meditation in the yogic path to liberation. The syllable is often found at the beginning and the end of chapters in the Vedas, the Upanishads, and other Hindu texts. It is described as the goal of all the Vedas.

Om emerged in the Vedic corpus and is said to be an encapsulated form of Samavedic chants or songs. It is a sacred spiritual incantation made before and during the recitation of spiritual texts, during puja and private prayers, in ceremonies of rites of passage (samskara) such as weddings, and during meditative and spiritual activities such as Pranava yoga. It is part of the iconography found in ancient and medieval era manuscripts, temples, monasteries, and spiritual retreats in Hinduism, Buddhism, Jainism, and Sikhism. As a syllable, it is often chanted either independently or before a spiritual recitation and during meditation in Hinduism, Buddhism, and Jainism.

The syllable Om is also referred to as Onkara (Omkara) and Pranava among many other names.

The syllable Om is referred to by many names, including:

The etymological origins of ōm (aum) have long been discussed and disputed, with even the Upanishads having proposed multiple Sanskrit etymologies for aum, including: from "ām" (आम्; "yes"), from "ávam" (आवम्; "that, thus, yes"), and from the Sanskrit roots "āv-" (अव्; "to urge") or "āp-" (आप्; "to attain"). In 1889, Maurice Bloomfield proposed an origin from a Proto-Indo-European introductory particle "*au" with a function similar to the Sanskrit particle "atha" (अथ). However, contemporary Indologist Asko Parpola proposes a borrowing from Dravidian "*ām" meaning "'it is so', 'let it be so', 'yes'", a contraction of "*ākum", cognate with modern Tamil "ām" (ஆம்) meaning "yes". In the Jaffna Tamil dialect spoken in Sri Lanka, aum' is the word for yes.^{[citation needed]}

Om emerged in the Vedic corpus and is said to be an encapsulated form of Samavedic chants or songs. The Samaveda, 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 Aitareya Brahmana of Rig Veda, in section 5.32, suggests that the three phonetic components of Om (a + u + m) correspond to the three stages of cosmic creation, and when it is read or said, it celebrates the creative powers of the universe. However, regardless of its original meaning, the syllable Om evolves to mean many abstract ideas even in the earliest Upanishads. Max Müller and other scholars state that these philosophical texts recommend Om as a basic tool for meditation and explain the various meanings that the syllable may hold in the mind of one meditating, ranging from "artificial and senseless" to the "highest concepts such as the cause of the Universe, essence of life, Brahman, Atman, and Self-knowledge".

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 Brahmana layer of Vedic texts equates Om with bhur-bhuvah-svah, which symbolizes "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 symbolic foundations of Om are repeatedly discussed in the oldest layers of the early Upanishads. In the Upanishads, it has been associated with various concepts, such as "cosmic sound", "mystical syllable", "affirmation to something divine", or as symbolism for abstract spiritual concepts. However, in the eight anuvaka of the Taittiriya Upanishad, which consensus research indicates was formulated around the same time or preceding Aitareya Brahmana, the sound Aum is attributed to reflecting the inner part of the word Brahman. Put another way, it is the Brahman, in the form of a word.

When occurring within spoken Classical Sanskrit, the syllable is subject to the normal rules of sandhi in Sanskrit grammar, with the additional peculiarity that the initial o of "Om" is the guṇa vowel grade of u, not the vṛddhi grade, and is therefore pronounced as a monophthong with a long vowel ([oː]), i.e. ōm not aum. Furthermore, the final m is often assimilated into the preceding vowel as nasalisation (raṅga). As a result, Om is regularly pronounced [õː] in the context of Sanskrit.