Upanishad means the inner or mystic teaching. The term Upanishad is derived from upa (near), ni (down), and s(h)ad (to sit), i.e., sitting down near. Groups of pupils sit near the teacher to learn from him the secret doctrine. In the quietude of the forest hermitages, the Upanishad thinkers pondered on the problems of deepest concerns and communicated their knowledge to fit pupils near them. Samkara derives the word Upanishad as a substitute from the root sad, ‘to loosen.,’ ‘to reach’ or ‘to destroy’ with Upa and ni as prefixes and keep as termination. If this determination is accepted, upanishad means Brahma-knowledge by which ignorance is loosened or destroyed. The treatises that deal with Brahma knowledge are called the Upanishads and so pass for the Vedanta. The different derivations together make out that the Upanishads give us both spiritual vision and philosophical argument. There is a core of certainty that is essentially incommunicable except by a way of life. It is by a strictly personal effort that one can reach the truth.
The Upanishads more clearly set forth the prime Vedic doctrines like Self-realization, yoga and meditation, karma, and reincarnation, which were hidden or kept veiled under the symbols of the older mystery religion. The older Upanishads are usually affixed to a particular Veda, through a Brahmana or Aranyaka. The more recent ones are not. The Upanishads became prevalent some centuries before the time of Krishna and Buddha.
The main figure in the Upanishads, though not present in many of them, is the sage Yajnavalkya. Most of the great teachings of later Hindu and Buddhist philosophy derive from him. He taught the great doctrine of “neti-neti“, the view that truth can be found only through the negation of all thoughts about it. Other important Upanishadic sages are Uddalaka Aruni, Shwetaketu, Shandilya, Aitareya, Pippalada, Sanat Kumara. Many earlier Vedic teachers like Manu, Brihaspati, Ayasya, and Narada are also found in the Upanishads.
In the Upanishads the spiritual meanings of the Vedic texts are brought out and emphasized in their own right.