Meaning of OM or AUM

Considered to be the most sacred and important of all sounds and mantras in the Hindu, Buddhist, and Jain traditions, oṃ is recited at the beginning and end of almost all prayers and meditational exercises. The word is understood to have three sounds, a, u, and m, with the diphthong au producing an o sound. The sound oṃ, which begins deep in the body and ends at the lips, is thought to be auspicious, and articulating it is said to exercise the whole vocal apparatus. The sound itself is variously considered to be a full manifestation of:

1.

brahman ;

2.

the universe (it is, in fact, coeval with the beginning of the universe); and

3.

the earth, atmosphere, and heaven.

Oṃ is frequently called

1.

praṇava

2.

mūlamantra (primary mantra), and

3.

akṣara – with which it has had a complex relationship (see van Buitenen, 1959).

Due to its sacredness, some sectarian Hindu traditions do not allow women and Śūdras to pronounce it. Since the late 20th century, oṃ has become disassociated with the religious traditions of its origin and is thought of as a spiritual symbol, and in this capacity, it has become commonplace in yoga studios in the West. Also in the diaspora, oṃ has become a symbol of all things Hindu, superseding other Hindu symbols such as the svastika or the word śrī, which are common in Hindu practice in the subcontinent. Thus, oṃ has come to represent Hinduism as the cross represents Christianity, and the Star of David the Jewish tradition. Speaking of its importance, J. Gonda says, "praṇava, i.e., the 'mystic' syllable oṃ – originally a 'numinous primeval sound' which is still uttered with the utmost reverence – is throughout many centuries regarded as a positive emblem of the Supreme. It is said to have flashed forth in the heart of Brahmā, while he was absorbed in deep meditation. It unfolded itself in the form of the Gāyatrī, which, in its turn, became the mother of the Veda’s [sic]" (Gonda, 1963, 274). 

Oṃ in the Vedas and Upaniṣads 

The origins of oṃ are not clear, but according to H.H. Hock (1991), it originated in the Yajurvedasaṃhitā, the Jaiminīyabrāhmaṇa, and the Aitreyabrāhmaṇa  and was thus part of the vedic tradition:  "as an exclamatory particle, a filler particle, or a recitational substitute for part of the utterance … [it] is a secondary development from its exclamatory filler and recitational uses; all of these overlap in the context of Vedic rituals" (quoted in Young, 2002, 84). 
In the Dharmasūtras and Gṛhyasūtras, oṃ is associated with the upanayana ritual and with the quest for salvation. Āpastambhadharmasūtra 1.4.13.6, for example, says that oṃ is the doorway to heaven.
However, it is in the Upaniṣads that oṃ is discussed at length. According to the Chāndogyopaniṣad, oṃ is identical to the udgītha (high chant): “the High Chant is OṂ and OṂ is the High Chant” (ChU. 1.5.1; Olivelle, 1996, 101). Chāndogyopaniṣad 1.4.4–5 states that a person who “enters” this syllable with the knowledge that the gods who knew it became immortal and free from fear will also become immortal and free from fear (Olivelle, 1996, 101). The Kaṭhopaniṣad (1.2.15–16) speaks of oṃ as being the entire cosmos. According to the Taittirīyopaniṣad (1.8.1–2), oṃ is the supreme brahman; to know it is to experience brahman. The Muṇḍakopaniṣad (2.2.4) uses the analogy of archery: oṃ is the bow, ātman the arrow, and brahman the target (Olivelle, 1996, 273). The Praśnopaniṣad (5.5–6) says that by meditating on the three phonemes (a, u, and m), a person is released from evil, “just like snake from his slough,” and becomes immortal (Olivelle, 1996, 285–286).
A. Padoux sums up the significance of the three phonemes by correlating them to different triads, such as the three Vedas (Ṛgveda, Sāmaveda, and Yajurveda), the three parts of the cosmos (earth, atmosphere, and heaven), and the three vital breaths, which play a role in the later tantric Mantramārga. He further adds in the Maitryopaniṣad that: "Om is the essence of everything in the human heart, that meditation on Brahman rests eternally upon it, and that when stirred up it rises to the throat as an atom of sound, then reaches the tip of the tongue, finally flowing out as speech.The person who sees all this, the Upaniṣad adds, sees only the Absolute and is freed from death and suffering" (Padoux, 2003, 480). 
The Māṇḍukyopaniṣad gives a detailed exposition of the meaning and power of oṃ. The whole world is contained in the syllable oṃ (MāṇḍU. 1; Olivelle, 1996, 289). Several Advaita Vedānta teachers, starting with Gauḍapāda, Śaṅkara’s guru, have commented extensively on the first verses of the Māṇḍukyopaniṣad. According to Gauḍapāda, each of the three sounds a, u, and m has its own specific experiential meaning:

1.

“a” stands for the world that we experience when awake (viśva), the person who is awake, and the waking experience.

2.

“u” stands for the dream world (taijasa), the dreamer, and the dream experience.

3.

“m” stands for the world of dreamless sleep, the sleeper, and the sleep experience (prajñā).

4.

In addition there is a fourth (turīya) meaning: that of the unspoken syllable. It represents that which cannot be described in words. It is beyond measure (amātra); it is the “boundless” that will be the “soundless” end to the word. In the words of G.L. Beck, “[t]he three sounds, AUM merge into the soundless Aum; here the three sounds become identical with it as the three sounds are identical with it from the Absolute standpoint” (Beck, 2003, 90–91). 

Oṃ in Later Literature 

The Manusmṛti (11.265–266) describes the essence of the Vedas as being found in the three letters of oṃ. The Yogasūtra and its commentaries also discuss the importance of oṃ/praṇava as a meditational tool: 
tasyavācakaḥ praṇavaḥ
tajjapa stadarthabhāvanam  
Oṃ is his [Īśvara’s] expression
The repetition of this [oṃ] [and] the realization of its meaning [result in ekāgratā, one-pointedness].
(YS. 1.27–28; trans. by author) 
The Yogaśikhopaniṣad (2.9) connects oṃ/praṇava with the lowest cakra, the mūlādhāracakra, and therefore calls it mūlamantra (Beck, 1993, 95). The importance of the praṇava has also been dealt with at length by Śrīvaiṣṇava theologians, such as Vedāntadeśika in his Rahasyatrayasāra, Piḷḷai Lōkācārya in his Mumukṣupadi Sārārtha Dīpikā, and Parāśara Bhaṭṭar in his Aṣṭaślokī:

1.

“a” represents brahman, understood here as Viṣṇu-Nārāyaṇa and Śrī-Lakṣṃī;

2.

“u” indicates the relationship between ātman and Viṣṇu-Nārāyaṇa, its sole master; and

3.

“m” indicates ātman (Parthasarathy, 1995, 35; Srinivasan, 2008, 28; Rajagopala Ayyangar, 1956, 351–363). 

However, other Vaiṣṇavas say that the sounds represent Viṣṇu, Śrī, and the devotee.

Regional Traditions 

There are many regional traditions that incorporate or coopt the significance of oṃ. In the pilgrimage town of Swamimalai, near Kumbakonam, Tamil Nadu, for instance, the local tradition narrates that the deity Murukaṉ expounded the meaning and importance of oṃ to Śiva, his father. In another regional story, Śiva and Pārvatī are said to build a boat from the sound oṃ and ride it to the root of the universe (Shulman, 1988, 297). The vimānam or the tower over the central shrine in some temples in South India, including the ones in Srirangam and Athanur, is called the praṇavavimānam, because it is supposed to embody and contain the importance of the oṃ mantra, concretizing the vocal and aural significance of the sound in architecture.

 
Fig. 1: Pilgrimage poster of the tīrtha Omkareshwar (“Lord of the Sound Oṃ”) along the river Narmadā in Madhya Pradesh. The island with the jyotirliṅga temple devoted to Śiva is supposed to be shaped like the sacred sound oṃ. 
 
In sectarian traditions, Viṣṇu, Śiva, and Devī are said to be embodiments of the sound oṃ. In popular discourse in South India, however, it is Gaṇeśa whose inherent form (svarūpa) is that of the praṇava. For example, the Vātāpi Gaṇapati, a well-known song composed by Muttusvāmi Dīkṣitar in the classical Carnatic music tradition, acknowledges Gaṇeśa as the svarūpa of the praṇava.
Oṃ is represented visually in several ways, and it is one of the most popular graphics in both Hindu and New Age movements. Represented like the numeral three with a little tail, it has a crescent and a dot on top.

Conclusion 

Hindus agree that oṃ is the most sacred sound, but they interpret it in multiple ways. In a sense, then, the sound oṃ is greater than the sum of its parts, exceeding in significance the many meanings attributed to it. Oṃ, however, is no longer just part of the Hindu, Buddhist, and Jain heritages; it has become an integral part of yoga practice in the whole world. Although attested to in the vedic corpus, it perhaps has never enjoyed as much popularity as it does in the current time.