Turīya Skt., lit. “the fourth”; the superconscious state of illumination, called the fourth because it transcends the three familiar states (avasthā) of waking, dreaming, and deep sleep. This state of absolute consciousness is beyond thought, causality, and identification with the body; it is indescribable. From the psychological point of view, it is called Turīya; its philosophical name is Brahman. The Māndūkya-Upanishad, which contains an analysis of the various states of consciousness, describes turīya as “neither subjective nor objective experience, neither knowledge of the senses, nor relative knowledge, nor derived knowledge.” It is positively described as “pure, unified consciousness, unspeakable peace” and “the nature of the Ātman” (See also brahmachaitanya.)
Source: The Encyclopedia of Eastern Philosophy and Religion: Buddhism, Hinduism, Taoism, Zen. Shambhala Publications, Inc.