Aum in the Mandukya Upanishad (an ancient text, consisting of twelve verses expounding the mystic syllable Aum) -
the three psychological states of waking, dreaming and sleeping, and the transcendent fourth state of illumination.
There are three "letters" in the word aum : ‘a’, ‘u’ and ‘m’.
The ‘a’ stands for the state of wakefulness, where we experience externally through our mind and sense organs.
The ‘u’ stands for the dream state, in which inward experiences are available.
In the state of deep sleep, represented by the sound ‘m’, there is no desire and consciousness is gathered in upon itself.
But there is a fourth, transcendent state, that of one "who is neither inwardly nor outwardly aware, nor both inward and outward, nor with consciousness infolded on itself.... who is unseen and ineffable, ungraspable, featureless, unthinkable and unnameable." The fourth state (turīya avasthā) corresponds to silence as the other three correspond to AUM. It is the substratum of the other three states. It is referred to as atyanta-shunyata (absolute emptiness)