Turiya, “the fourth,” is the name Vedanta gives to what is left when the three ordinary states of consciousness are accounted for and set aside. Waking, dream, and dreamless sleep, jagrat, svapna, and sushupti, come and go in cycles, each replacing the last. Turiya is not a fourth state alongside these three, arriving and departing in its own turn. It is the constant awareness in which all three appear, the screen on which waking, dreaming, and sleeping are projected in succession, itself never projected and never absent.
The Mandukya Upanishad, the shortest of the principal Upanishads, is devoted almost entirely to unfolding turiya through the syllable Om. Each of the three sounds of Om, a, u, and m, corresponds to waking, dream, and deep sleep. But the silence that follows the sound, the fourth measure with no sound of its own, corresponds to turiya. It is not one more piece of the syllable; it is what makes the whole utterance possible and what remains when the utterance ends. In the same way, turiya is not one more experience alongside others; it is the condition for there being any experience at all.
This is why turiya is described as the true nature of the Self rather than a state to be attained through effort. Waking, dream, and sleep all come with a sense of a subject encountering objects, even if in sleep the objects are absent and only ignorance remains. Turiya has no such division. It is sometimes called caturtha, simply “the fourth,” but also simply the Self, atman, since nothing needs to be added to it and nothing can be taken away. Realizing turiya is not leaving waking life for a special trance; it is recognizing the awareness already present in all three states as one’s own unchanging nature.