OM!

From Wikipedia.

Aum (also Om, written in Devanagari as ?, in Chinese as ?, in Tibetan as ?, in Sanskrit known as pra?ava ????? lit. "to sound out loudly" or o?k?ra ????? lit. "o? syllable") is a mystical or sacred syllable in the Indian religions, including Hinduism, Sikhism, Jainism, and Buddhism.

Aum is pronounced as a long or over-long nasalized close-mid back rounded vowel, [õ??]). It is placed at the beginning of most Hindu texts as a sacred exclamation to be uttered at the beginning and end of a reading of the Vedas or previously to any prayer or mantra. The Mandukya Upanishad is entirely devoted to the explanation of the syllable. The syllable is taken to consist of three phonemes, a, u and m, variously symbolizing the Three Vedas or the Hindu Trimurti or three stages in life ( birth, life and death )

The name omkara is taken as a name of God in the Hindu revivalist Arya Samaj. Similarly, the concept of om, called onkar in Punjabi, is found in Sikh theology as a symbol of God. It invariably emphasizes God's singularity, expressed as Ek Onkar ("One Omkara" or "The Aum is One"), stating that the multiplicity of existence symbolized in the aum syllable is really founded in a singular God.[1]