The Upanishads (Devanagari: उपनिषद्, IAST: upaniṣad) are Hindu scriptures in Sanskrit language. They contain the central religious concepts of Hinduism, some of which are shared with Buddhism and Jainism. The Upanishads are considered by the Hindus as Sruti, or that "which is heard". The early Upanishads discuss the nature of ultimate reality (brahman), Ātman (Soul, Self), Self-knowledge, and the means for human salvation (Moksha), freedom and a content, happy life in Hinduism.
The Upanishads are the foundation of Hindu philosophical thought and its diverse traditions. Of the Vedic corpus, they alone are widely known, and the central ideas of the Upanishads are at the spiritual core of Hindus.
More than 200 Upanishads are known, of which the first dozen or so are the oldest and most important and are referred to as the principal or main (mukhya) Upanishads. The mukhya Upanishads are found in the Vedas. The oldest, such as the Brhadaranyaka and Chandogya Upanishads, have been dated to the early half of the first millennium BCE. The early Upanishads all predate the Common Era, some in all likelihood pre-Buddhist (6th century BCE), down to the Maurya period of ancient Indian history. Of the remainder, some 95 Upanishads are part of the Muktika canon, composed from about the start of common era through medieval Hinduism. New Upanishads, beyond the 108 in the Muktika canon, continued to being composed through the early modern and modern era, some dealing with subjects which have little to no connection to the Vedas.