Turiya Wikipedia.

 “In the Mandukya Upanishad, AUM (Om) symbolizes the four states of conciousness. The three parts in A-U-M corresponds to the waking, dreaming, and sleep states. The fourth state corresponds to silence, it is the substratum of the other three states. It is, states Nakamura, atyanta-shunyata (absolute emptiness). According to Sharma, Turiya is "the common ground of all these states. It manifests itself in these three states and yet in its own nature it transcends them all".” - Wikipedia.