Upanishad is formed from three words meaning "near" "down" and "sit" and refers to the act of students sitting close to a teacher who is imparting some important or secret doctrine.
Sanskrit pundit Vasant Lad says, "The Upanishads are the later, higher teachings of the Vedas; implies sitting in the vicinity of an enlightened one (Guru) and listening to him or her without any doubt, delusion, or comparison. The entire teaching of Vedanta is Upanishad. Upa-Vedas - Secondary or subordinate Vedas. Ayurveda is an upa-Veda."
The Upanishads are one part of Hindu scriptures, describing reincarnation. Some centuries before the time of Krishna and Buddha the Upanishads were a dominant teaching. The sage Yajnavalkya is said to be the primary teacher of the Upanishads, which are written in Sanskrit. It is believed that later Hindu and Buddhist philosophy is founded on Upanishad teachings.
The core spiritual vision of the Upanishads is a philosophical argument that says only through a devoted personal effort can one find truth. Vedic doctrines such as self-realization, yoga and meditation, karma and reincarnation veiled in mystery in older religion but the Upanishads reveal much of this mystery by affixing a particular Veda through a Brahmana or Aranyaka. The Upanishads espouse neti-neti, a view that truth can be found only through the negation of all thoughts around truth. 
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