Sometimes referred to as Adi ("the first") Shankaracharya (Shankara + acharya, "teacher"); India's most illustrious philosopher. His date is uncertain; many scholars assign him to the eighth or early ninth century. He expounded God not as a negative abstraction, but as positive, eternal, omnipresent, ever new Bliss.
Shankara reorganized the ancient Swami Order, and founded four great maths (monastic centers of spiritual education), whose leaders in apostolic succession bear the title of Jagadguru Sri Shankaracharya. The meaning of Jagadguru is "world teacher."
Further reading: Commentary on the Upanishads