Om - The Primordial Sound
The sacred syllable representing ultimate reality.
Om (Aum) is the sound-form of Brahman. It encompasses all sounds, all mantras, all knowledge. Meditation on Om leads to realization of the Self.
Om is the bow, the Self is the arrow, Brahman is the target.
Om is considered the most powerful of all mantras.
Mandukya Upanishad