Om is frequently called the pranava, literally "humming," a word that derives from pranu, "to reverberate," and ultimately from the root nu, "to praise or command" but also "to sound or shout." It is the audible expression of the transcendental, attributeless ground of reality.
Om is the "primordial seed" of the universe--this whole world, says one ancient text, "is nothing but om." It is also considered to be the root mantra from which all other mantras emerge and to encapsulate the essence of the many thousands of verses of Hinduism's holiest texts, the Vedas. According to the Katha Upanishad (2.15), om is the "word which all the Vedas rehearse."
As such, om is the meditative seed par excellence. Patanjali--who wrote the Yoga Sutra and is considered to be the father of classical yoga--taught that when we chant this sacred syllable and simultaneously contemplate the meaning of it, our consciousness becomes "one-pointed: and prepared for meditation. In a commentary on the Yoga Sutra, the ancient sage Vyasa noted that through chanting om, "the supreme soul is revealed." In a similar vein, Tibetan scholar Lama Govinda wrote that om expresses and leads to the "experience of the infinite within us." Thus, chanting om may be the easiest way to touch the Divine within your very self.
Yogis often meditate on the four "measures," or parts, of om. Though commonly spelled om, the mantra actually consists of three letters, a, u, and m. (In Sanskrit, whenever an initial a is followed by a u, they coalesce into a long o sound.) Each of these three parts has numerous metaphysical associations, which themselves serve as meditative seeds. For example, a (pronounced "ah") represents our waking state, which is also the subjective consciousness of the outer world; u (pronounced "ooh") is the dreaming state, or the consciousness of our inner world of thoughts, dreams, memories, and so on; and m is the dreamless state of deep sleep and the experience of ultimate unity.
http://www.yogajournal.com/wisdom/1141
