The Upanishads are commonly referred to as Ved?nta, 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) andtman (Soul, Self) are central ideas in all the Upanishads, and "Know yourtman" 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. 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 orally. 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 last c . . . more

https://en.wikipedia.org/wiki/Upanishads