The term Upanishad is a Sanskrit word, which means "sitting down beside". The meaning of Upanishad if simply translated is, "sitting very close to the teacher and sharing." In ancient time, students used to sit near the teacher for learning hence the term Upanishad brings back the memoir of the days long gone. However, as per the native authorities, Upanishad means, "setting at rest ignorance by revealing the knowledge of the supreme spirit". The term Upanishad has come from `Upa` which means `near`, `ni` meaning `down`, and `sad` meaning `to sit`, which refers to the "sitting down near" a spiritual teacher or `guru` to get teaching in the `Guru-shishya tradition`. However, the dictionary meaning of the term is `secret doctrine`.

The exact date for the composition of Upanishads cannot be said. The origin of Upanishad is therefore deeply rooted in the antiquity. It is said that with the growth of the caste system and with the introduction of varied styles of worship, the Kshatriyas 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. In the forest the Kshatriyas contemplated on the Vedas and from their experience they penned down the `Aranyakas`. These Aranyakas later turned into Upanishad.

Although, the exact dates cannot be assigned yet it is believed that Upanishads, are undoubtedly the oldest and the most authoritative in the Indian philosophy. The earliest of the Upanishads date back to the pre-Buddhistic era and there are a few Upanishads that were written after 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. The popular religious teacher, Samkara did make his comments on about ten of the Upanishads that were composed at a later period, during the pre-Buddhistic period of about 400 or 300 B.C. However eleven principal Upanishads has been recognised. These are Katha Upanishad, Isa Upanishad , Kena Upanishad , Mundaka Upanishad , Svetasvatara Upanishad , Prasna Upanishad , Mandukya Upanishad , Aitareya Upanishad, Brihadaranyaka Upanishad , Taittiriya Upanishad and Chhandogya Upanishad. Ramanuja uses all these Upanishads as the Subdha and Culika. He also talks about the Garbha, the Jbdha and the Mahd Upanishads. 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, Yajfiavalkya, Balaki, Svetaketu, and Sindilya are associated with the chief doctrines of the Upanishads.

The Upanishads are considered as part of the Vedas and thus 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. Also known as the Vedanta, (the end of the Vedas), the Upanishad does not belong to any particular period of Sanskrit literature. The Upanishads are essentially a set of ancient mystic teachings. In the ancient Indian philosophy, the Upanishads teamed with the philosophy of Upanishad are always regarded as a class of literature that is independent of the Vedic hymns and the Brahmanas.

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 and those conceptions were quite deeply rooted in the Indian consciousness. This root can easily be overthrown and the Upanishads subordinated the concept of many Gods, to the One. Upanishad is therefore not the theory of science or just the reflection of philosophy, but the doctrine for right living. Upanishad aims to liberate the spirit from the trammels of the flesh that it might enjoy communion with God. Apart from having the feeling of reverence for the past, the thinkers of Upanishads also made the intellectual discipline, as subsidiary to holiness of life. The Upanishads actually sought to square a growing idealistic philosophy with the dogmas of a settled theology, through this way. A compromise between the philosophic faith of the few and the fancied superstition of the crowds was thus neatly reconciled in the philosophy and mysticism of the Upanishad

The idea of rationalism is also deeply seated in the philosophy of Upanishad. The very kernel theory of rationalism, in Upanishad points to an instinctive understanding of nature and natural processes where the ideas are presented in deep philosophical and exploratory manner whilst hinting the rational approach of Indian philosophy.