Turiya, Sanskrit for the fourth, names a state of consciousness described most concisely in the short Mandukya Upanishad, which maps the syllable Om onto four states of the self: waking consciousness, dreaming consciousness, deep dreamless sleep, and a fourth state, turiya, that underlies and is present within the other three without itself being another content of experience alongside them. The Mandukya describes turiya negatively, not this and not that, as neither inwardly nor outwardly cognitive, neither a mass of cognition nor simple absence of cognition, unseen, ungraspable, without distinguishing marks, the very self that must be realized rather than an object that can be known the way waking experience knows its objects.
Because turiya is described as continuously present beneath the other three states rather than as an additional experience that comes and goes, later Vedantic commentary treats it not as a fourth item on a list but as the unchanging ground of awareness itself, the witness, sakshi, that is present whether the mind is awake, dreaming or asleep, distinguishing turiya sharply from meditative attainments such as samadhi that must be entered and left. Shankara's commentary on the Mandukya, transmitted through his teacher's teacher Gaudapada, treats this identification of turiya with the self as the Upanishad's single most important teaching.