OM/AUM

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.