The term Upanishad literally means, 'sitting down near' or 'sitting close to', and implies listening closely to the mystic doctrines of a guru or a spiritual teacher, who has cognized the fundamental truths of the universe. It points to a period in time when groups of pupils sat near the teacher and learnt from him the secret teachings in the quietude of forest 'ashrams' or hermitages. In another sense of the term, Upanishad means brahma-knowledge by which ignorance is annihilated. Some other possible meanings of the compound word Upanishad are placing side by side (equivalence or correlation), a near approach (to the Absolute Being), secret wisdom or even sitting near the enlightened.
The Upanishads more clearly set forth the prime Vedic doctrines like self-realisation, 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 particularly Veda, through a Brahmana or Aranyaka. The Upanishads became prevalent some centuries before the time of Krishna and Buddha.
The Upanishads form the core of Indian philosophy. They are an amazing collection of writings from original oral transmissions. It is here that we find all the fundamental teachings that are central to Hinduism — the concepts of 'karma' (action), 'samsara' (reincarnation), 'moksha ' (nirvana), the ' atman ' (soul), and the 'Brahman' (Absolute Almighty). They also set forth the prime Vedic doctrines of self-realisation, yoga and meditation. The Upanishads are summits of thought on mankind and the universe, designed to push human ideas to their very limit and beyond. They give us both spiritual vision and philosophical argument, and it is by a strictly personal effort that one can reach the truth.