The Upanishads (/uːˈpænɪˌʃædz, uːˈpɑːnɪˌʃɑːdz/; singular: IAST: Upaniṣat, IPA: [upəniʂət̪]) are a collection of texts in the Vedic Sanskrit language which contain the earliest emergence of some of the central religious concepts of Hinduism, some of which are shared with Buddhism and Jainism. The Upanishads are considered by Hindus to contain revealed truths (Sruti) concerning the nature of ultimate reality (brahman) and describing the character and form of human salvation (moksha).
The Upanishads are sometimes referred to as Vedanta, variously interpreted to mean either the "last chapters, parts of the Veda" or "the object, the highest purpose of the Veda". The concepts of Brahman (Ultimate Reality) and Ātman (Soul, Self) are central ideas in all the Upanishads, and "Know your Ātman" their thematic focus. 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.
Example via www.ramdass.org: “The Upanishads: In The Light of Kriya Yoga”
Example via www.mindpodnetwork.com: Sound & Silence
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 mostly in the concluding part of the Brahmanas and Aranyakas and were, for centuries, memorized by each generation and passed down verbally. The early Upanishads all predate the Common Era, some in all likelihood pre-Buddhist (6th century BCE), down to the Maurya period. 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, though often dealing with subjects which are unconnected to the Vedas.
Along with the Bhagavad Gita and the Brahmasutra the mukhya Upanishads (known collectively as the Prasthanatrayi), provide a foundation for the several later schools of Vedanta, among them, two influential monistic schools of Hinduism.
With the translation of the Upanishads in the early 19th century they also started to attract attention from a western audience. Schopenhauer was deeply impressed by the Upanishads and called it "the production of the highest human wisdom". The 19th century transcendentalists noted the influence of the Upanishads in western philosophy.