upaniṣad
The definite, well ascertained knowledge that is self, which makes one recognize brahman and destroys all sorrow; revealed texts dealing with the self; that which is the most profound secret.
In Vedanta
The end portions of the Vedas that reveal the nature of ātmā and Brahman. The word is traditionally derived as upa-ni-ṣad — that which destroys (ṣad) ignorance by bringing the student near (upa-ni) to the truth. The principal Upaniṣads commented upon by Śaṅkara form the prasthānatraya along with the Brahmasūtras and the Bhagavadgītā.