Upanishad literally means sitting near devotedly, and brings to mind the picture of a sincere devotee learning from his guru or spiritual master. The great Sankaracharya would, however, describe the Upanishad as simply knowledge of God.
There are said to be 108 Upanishads ranging in length from a few hundred to many thousands of words, some in prose and some in verse. Of these, sixteen were recognized by Sankaracharya as authentic and authoritative. In this commentary on Vedanta Aphorisms, he included quotations from six. On the other ten, he wrote elaborate commentaries. It is these ten which, partly on account of their intrinsic importance but mainly, no doubt, because of Sankara’s commentaries, have come to be regarded as the principle Upanishads. These ten Upanishads that form one of the vital bases of Hinduism are Isa, Kena, Katha, Prasna, Mundaka, Mandukya, Chandogya, Brihadaranyaka, Aitareya and Taittiriya.