aum, also 
om (Devanagari: ॐ) is the most sacred syllable in Hindu Dharma, first coming to light in the Vedic Tradition. The character is a composite of three different letters of the 
Sanskrit alphabet. The syllable is sometimes referred to as the 
udgitha or 
pranava mantra (primordial mantra); not only because it is considered to be the primal sound, but also because most mantras begin with it. In 
Devanagari it is written ॐ, and in 
Tibetan script ༀ.
"Aum" is the most sacred syllable often spoken during the practice of
 any Hindu rites. It is a holy character of the Sanskrit language, the 
language of God. The character is a composite of three different letters
 of the Sanskrit alphabet. Because of its significance this sacred 
syllable is spoken before any chants to show God we remember him. This 
sign in Hinduism also represents the whole universe.
The Significance of the Symbol ॐ
The symbol ॐ (
aum, also called pranava),
 is the most sacred symbol in Hinduism. Volumes have been written in 
Sanskrit illustrating the significance of this mystic symbol. Although 
this symbol is mentioned in all the Upanishads and in all Hindu 
scriptures, it is especially elaborated upon in the Taittiriya Upanishad, Chandogya Upanishad and Mundaka Upanishad Upanishads.
The
 goal, which all Vedas declare, which all austerities aim at, and which 
humans desire when they live a life of continence, I will tell you 
briefly it is aum. The syllable aum is indeed Brahman. This syllable aum
 is the highest. Whosoever knows this symbol 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 Brahman.
—Katha Upanishad I, ii, 15-17
 
The symbol of ॐ (
aum) contains of three curves, one 
semicircle and a dot. The large lower curve symbolizes the waking state;
 the upper curve denotes deep sleep (or the unconscious) state, and the 
lower curve (which lies between deep sleep and the waking state) 
signifies the dream state. These three states of an individual’s 
consciousness, and therefore the entire physical phenomenon, are 
represented by the three curves. The dot signifies the Absolute (fourth 
or Turiya state of consciousness), which illuminates the other three 
states. The semicircle symbolizes maya
 and separates the dot from the other three curves. The semicircle is 
open on the top, which means that the absolute is infinite and is not 
affected by maya. 
Maya only affects the manifested phenomenon. In this way the form of 
aum symbolizes the infinite Brahman and the entire Universe.
Uttering
 the monosyllable ॐ, the eternal world of Brahman, One who departs 
leaving the body (at death), he attains the superior goal.
— Bhagavad Gita, 8.13
 
ॐ (AUM) — Origin
Found first in the Vedic scriptures of Hinduism, 
aum has 
been seen as the first manifestation of the unmanifest Brahman (the 
single Divine Ground of Hinduism) that resulted in the phenomenal 
universe. Essentially, all the cosmos stems from the vibration of the 
sound 'Aum' in Hindu cosmology. Indeed, so sacred is it that it is 
prefixed and suffixed to all Hindu mantras and incantations. It is 
undoubtedly the most representative symbol of Hinduism.
Although the ॐ symbol's left part, ऊ, which looks like a figure 3, looks like the uu vowel in the 
Devanagari script, specifically, when used as a syllable with no attached initial consonant, it is actually based on 
Brahmi version of ओ. The nasal sound is indicated by a 
chandrabindu.
Philosophy of AUM
Gods and Goddesses are sometimes referred to as Aumkar
 (or Omkar), which means form of Aum, thus implying that who are 
limitless, the vibrational whole of the cosmos. Ek Onkar, meaning 'one 
god' is a central tenet of Sikh religious philosophy. In Hindu 
metaphysics, it is proposed that the manifested cosmos (from Brahman) 
has name and form (nama-rupa), and that the closest approximation to the
 name and form of the universe is Aum, since all existence is 
fundamentally composed of vibration. (This concept of describing reality
 as vibrations, or rythmic waves, can also be found in quantum physics 
and super string theory, which describe the universe in terms of 
vibrating fields or strings.)
In advaita philosophy 
it is frequently used to represent three subsumed into one, a common 
theme in Hinduism. It implies that our current existence is mithya, or 
'slightly lesser reality,' that in order to know the full truth we must 
comprehend beyond the body and intellect and intuit the true nature of 
infinity, of a Divine Ground that is imminent but also transcends all 
duality, being and non-being, that cannot be described in words. Within 
this metaphysical symbolism, the three are represented by the lower 
curve, upper curve and tail of the ॐ subsumed into the ultimate One, 
represented by the little crescent moon-shape and dot, known as 
chandrabindu. Essentially, upon moksha, mukti, samadhi, nirvana, 
liberation, etc. one is able not only to see or know existence for what 
it is, but to become it. In attaining truth one simply realizes 
fundamental unity; it is not the joining together of a prior manifold 
splitting. When one gains true knowledge, there is no split between 
knower and known: one becomes knowledge/consciousness itself. In 
essence, Aum is the signifier of the ultimate truth that all is one.
For the scriptural esoteric explanation of Aum see Mandukya Upanishad.
dvaita-advaita 
(Vaishnava) philosophies teach that 'Aum' is an impersonal sound 
representation of Vishnu/Krishna while Hari Nama is the personal sound 
representation. A represents Krishna, U Srimati Radharani and M jivas. 
According to Sridhara Svami the pranava has five parts: A, U, M, the 
nasal bindu and the reverberation (nada). Liberated souls meditate on 
the Lord at the end of that reverberation.
Examples of Three into One:
- Creation (Brahma)- Preservation (Vishnu)- Destruction (Shiva) into Brahman
 
- Waking- Dreaming- Dreamless Sleep into Turiya (transcendental fourth state of consciousness)
 
- Rajas (activity, heat, fire)- Tamas (dullness, ignorance, darkness)- Sattva (purity, light, serenity/shanti) into Brahman
 
- Body, Speech and Mind into Oneness
 
The Chandogya Upanishad (1.1.1-10) states,
"The udgitha is the best of all essences, the highest, deserving the highest place, the eighth."
"Aum" can be seen as Sri Ganesh, whose figure is often represented in
 the shape of Aum. He is thus known as Aumkar (Shape of Aum). Sri 
Nataraja, or the Hindu god 'Shiva' dancing his dance of destruction, is 
seen in that popular representation mirroring the image of Aum. It is 
said to be the most perfect 'approximation' of the cosmic existence 
within time and space, and therefore the sound closest to Truth.
"The First Word Om (Aum) It is also called Pranav because its sound 
emanates from the Prana (vital vibration), which feels the Universe. The
 scripture says "Aum Iti Ek Akşara Brahman" (Aum that one syllable is 
Brahman)."
The AUM Sound
"A - emerges from the throat, originating in the region of the navel U
 - rolls over the tongue M - ends on the lips A - waking, U - dreaming, M
 - sleeping It is the sum and substance of all the words that can 
emanate from the human throat. It is the primordial fundamental sound 
symbolic of the Universal Absolute.".
In fact, when correctly pronounced, or rather, "rendered", the "A" 
can be felt as a vibration that manifests itself near the navel or 
abdomen; the "U" can be felt vibrating the chest, and the "M" vibrates 
the cranium or the head. The abdominal vibration symbolises Creation; It
 is interesting that the "creative" or reproductive organs are also 
located in the lower abdomen. The vibration of the chest represents 
Preservation, which is also where the lungs are situated (the lungs 
sustain or preserve the body through breath). The vibration of the head 
is associated with Destruction or sacrifice, since all that gives up or 
destroys is first destroyed mentally. Hence, the entire cycle of the 
universe and all it contains is said to be symbolised in AUM.
Today, in all Hindu art and all over India and Nepal, 'Aum' can be 
seen virtually everywhere, a standard sign for Hinduism and a vast but 
economical storehouse for the deep mythology inherent in the world's 
oldest religion.
Notes the Chandogya Upanishad, "That syllable, is a syllable of 
permission; for, whenever we permit anything, we say Aum." However, this
 is seen by others as a myopic perspective because the same Hindu 
scriptures, the Upanishads, that aver this function also attribute to it
 the divine property of the source of the universe. Aum is seen as the 
source of existence as we know it within the causal dimensions of time 
and space, and thus affirmatory meanings in languages are a natural 
progression. Aum is not only affirmation, but negation, and transcends 
both.
The AUM sound is sometimes called "the 3-syllable Veda". The third 
syllable arises because in Devanagari and similar alphabets, a consonant
 at the end of a word is sometimes written as a separate consonant 
letter with the virama "no vowel" sign, and this combination is treated 
as a syllable when talking about Devanagari writing rather than about 
phonetics.
The Sanskrit word omkāra (from which came Punjabi onkār, etc), literally "OM-maker", has two families of meanings:-
Brahma (god) in his role as creator, and thus a word for "creator". Writers' term for the OM sign.
The use of the universal AUM sound can be found on every nearly 
continent and in many different cultures. The sound is unique to every 
individual: hence it can only be described by how it feels. Below is a 
secular guide to making the sound:
An individual's "Aum" is the sound that can be held steady the 
longest per breath for the longest consecutive sequence of breaths. It 
is called "aum" in every culture that is aware of it because it sounds 
like that in all humans. The lower pitches are more suited because they 
require less muscular contraction of the abdomen, leading to lower rates
 of oxygen consumption, allowing for longer time between breaths. The 
Aum is the exact sound that is easiest for the individual to produce.
Once the minimization of oxygen consumption occurs (by minimization 
of muscular exertion), the outflow of air will be steady and quite 
sensitive to any forces that alter the amount of pressure in the chest 
cavity. One of the most notable consequences of this is that the rythmic
 contractions of the heart become audible within the Aum.
Thus, by the use of Aum:
- one can easily hear their own heart.
 
- a person can modify the pace of their heart.
 
- a group of people can synchronize their heartbeats.