The Upanishads (Sanskrit: उपनिषद्, IAST:Upaniṣad, IPA: [upəniʂəd]) are philosophical texts considered to be an early source of Hindu religion. More than 200 are known, of which the first dozen or so, the oldest and most important, is variously referred to as the principal, main (mukhya) or old Upanishads. The oldest of these, the Brihadaranyaka, Jaiminiya Upanisadbrahmana and the Chandogya Upanishads, were composed during the pre-Buddhist era of India,[note 1] while the Taittiriya, Aitareya and Kausitaki, which show Buddhist influence, must have been composed after the 5th century BC. The remainder of the mukhya Upanishads are dated to the last few centuries BCE. New Upanishads were still composed in the medieval and early modern period: discoveries of newer Upanishads were being reported as late as 1926. One, the Muktikā Upanishad, predates 1656 and contains a list of 108 canonical Upanishads, including itself as the last. However, several texts under the title of “Upanishads” originated right up to the first half of the 20th century, some of which did not deal with subjects of Vedic philosophy. The newer Upanishads are known to be imitations of the mukhya Upanishads.
Definition courtesy Wikipedia.