Definition of Upanishads:
Each of the more than 200 sacred Hindu books written in Sanskrit is known as the Upanishad, of which the oldest and most representative would date from approximately between 800 and 400 BC. c.
The Upanishads are based on the mystical experiences of people who, tired of the official religion, retired to the forests to live as ascetics or hermits, thought for themselves and then spread their ideas.