Upanishads comes from the Sanskrit word Upa-ni-shad meaning to sit beside. In ancient times, students used to sit near the teacher for learning hence the term Upanishad. The dictionary meaning of the term is `secret doctrine`. The Upanishads are a part of the Hindu scriptures. Upanishad represents the deep Indian philosophy, the aura of meditation, the halo of religion and the indeed the nature of God. The Upanishads teamed with the philosophy of Vedanta are always regarded as a class of literature that is independent of the Vedic hymns and the Brahmanas.

Origin of Upanishads
The origin of Upanishad is deeply rooted in antiquity. It is said that with the growth of the caste system and with the introduction of varied styles of worship, the Kshatriya started rebelling against the usual practice of the priests. The Kshatriyas believed that most of the worship styles were corrupt and can never lead to the true path of self development. To gain knowledge and indeed to figure out the right path of self development the Kshatriyas then went into the forest to find the Rishis whom they believed to guide them towards the right path. They are undoubtedly the oldest and the most authoritative sources of Indian philosophy. The earliest of the Upanishads date back to the pre-Buddhist era and there are a few Upanishads that were written after the coming of Lord Buddha. The most possible time for the composition of those Upanishads is in between the completion of the Vedic hymns and the rise of Buddhism, in the sixth century B.C. There are also some dates available for the earliest Upanishads and the accepted dates for these Upanishads are 1000 B.C to 300 B.C.

Types of Upanishads
However eleven principal Upanishads has been recognised. These are Katha Upanishad, Isa Upanishad, Kenopanishad, Mundaka Upanishad, Svetasvatara Upanishad, Prashnopanishad, Mandukya Upanishad, Aitareya Upanishad, Brhadaranyaka Upanishad, Taittiriya Upanishad and Chandogya Upanishad. The names of the authors of Upanishad are not known since all the early literature of India was anonymous. The names of renowned sages like Aruni, Yajnavalkya, Balaki, Svetaketu and Sindilya are associated with the chief doctrines of the Upanishads.

Philosophy of Upanishads
The main philosophy of the Upanishads can be put into 3 main principles:
# There is one unifying principle behind the world.
# Theory of Karma is the basis of all Indian philosophy
# Material world is never a source of happiness and permanent peace.

The philosophy of Upanishad presented some polytheistic conceptions. The Upanishads subordinated the concept of many Gods, to the One. Upanishad is the doctrine for right living. It aims to liberate the spirit from the restrains of the flesh that it might enjoy communion with God. A compromise between the philosophic faith of the few and the superstition of the crowds was thus neatly reconciled in the philosophy and mysticism of the Upanishad.