OM, AUM
The most sacred single syllable in the entire literature of Hinduism is OM. This word is constantly on the lips of devout Hindus from the cradle to the grave. It stands for Brahman, both as personal and impersonal God.

Aum -Let us meditate on this syllable, which is the foundation of prayer.

The earth emerges from the waters; plants emerge from the earth; human beings emerge from plants; speech comes from human beings; and Aum comes from speech. Aum is the essence of all essences. It is the highest of the high. It is the ultimate.
Speech and breath combine to form Aum. Speech and breath are like a couple in coitus, and Aum is their offspring. Aum is the fulfillment of their desires.
The syllable Aum signifies assent, for we say it when we assent to something. And assent is nothing but fulfillment.
The syllable Aum is the call to knowledge. We sing the praises of this syllable, which is the key to every kind of knowledge.
Those who know the soul, and those who do not know the soul, stand side by side reciting this syllable. But knowledge and ignorance are quite different. Only when it is recited with knowledge and faith, and with awareness of the hidden connections, does it become truly potent.
Chandogya Upanishad I: I.
Omkara (pranava) is considered to be the sound incarnation of the Supreme Personality of God and is identical with the Supreme Lord. The Narada-pancaratra states: "When the transcendental sound vibration is practiced by a conditioned soul, the Supreme Lord is present on his tongue."
The Atharva-veda and the Mandukya Upanishad both mention the importance of omkara. Omkara is said to be the beginning, middle, and end, and is eternal, beyond all material restrictions. Omkara is unlimited, transcendental, and indestructible.
OM represents the wholeness of the universe and the eternal vibratory sound of brahman (the universal soul) that permeates it. Indeed, Om is called shabda-brahman, the sound-form of brahman. All Hindu mantras generally begin and often also end with Om. It has been frequently mentioned in the Vedas and other scriptures of Hinduism.
Om is the mantra of assent. It means yes and affirms and energizes whatever we say after it. That is why all mantras begin with OM. OM is also the mantra of ascent and causes our energy to rise upward into the infinite. By reciting Om, a person gains access to the powers of the universe; and meditation on Om is said to lead to enlightenment and immortality.
OM is expansive and increases the fire, air and ether elements, particularly ether. It also gives strength, protection and grace. It connects us with the guidance power of the inner Guru.
Aum is the supreme symbol of God. Aum is the whole. Aum affirms. Aum is the sound which expresses all truth.
The priest begins worship with Aum. Spiritual teachers begin their lessons with Aum; and their disciples open themselves to those lessons with Aum.

Those in whom Aum resides are unified with God.
Taittiriya Upanishad 1:8
The syllable Om, also known as Pranava, can also be spelled as AUM. According to one interpretation,

"A" stands for creation,
" U" stands for preservation, and
" M" indicates destruction or dissolution.
According to another interpretation, the three letters forming AUM indicate the three lokas (planes of existence) of this universe-both gross and subtle-Svarga (heaven), Martya (earth) and Patala (netherworld).
The following brief passage is one of the clearest of the countless references to OM which can be found anywhere in the Hindu scripture.
The goal which all the Vedas declare, which all austerities aim at, and which men desire when they lead the life of continence, I will tell you briefly: it is OM. This syllable OM is indeed Brahman.
This syllable is the Highest. Whosoever knows this syllable obtains all that he desires. This is the best support; this is the highest support. Whosoever knows this support is adored in the world of Brahma.
Katha Upanishad I, ii, 15-17
Om reveals all and contains all and, like the Vedas, Om is revelation that comes through shruti (hearing).
According to the Mandukya Upanishad, Om is both atman and brahman: it is the past, the present and the future, as well as all that transcends time.
The chanting of OM is generally practiced by impersonalists and those engaged in the mystic yoga process. By chanting OM and controlling the breathing perfectly, which is mostly a mechanical way of steadying the mind, one is eventually able to go into trance or samadhi. Through this system, one gradually changes the tendencies of the materially absorbed mind and makes it spiritualized. This process may take many years to perfect.
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