Turīya is the name the Māṇḍūkya Upaniṣad gives to the self beyond the three states of waking, dreaming and dreamless sleep. It is not a fourth state alongside them but the awareness in which all three come and go: peaceful, nondual, beyond description. Gauḍapāda’s commentary on the Upaniṣad made it central to Advaita.
Turīya
“The fourth”: pure consciousness beyond waking, dreaming and deep sleep, in the Māṇḍūkya Upaniṣad.
Also: Turiya, the fourth