School of Hindu philosophical doctrine formed in opposition to the concept of dualism. Its first proponent, Sankaracarya, drew upon the Upanishads and the Bhagavad-Gita. The basic tenet of Advaita-Vedanta is that there is a complete essential identity between brahman and Atman.
A poetic rendering of the concept of non-duality reads:
"We see lightness and darkness,
We hear sound and silence,
We feel pleasure and pain,
We taste sweetness and sourness,
We smell odour and freshness,
We know good and bad,
We experience in differences.
All are subjective. All we see, hear, feel, taste, smell, know, experience, ... we interpret as objects, where we ourselves are subject.
We are concerned perhaps too much with what we are conscious of, and forget the miracle of consciousness itself. A flow from many rivers into the ocean."
The idea of non-duality is also critical to the school of Buddhism known as Tantra, which combines the religion of Amitabha Siddhartha Gautama with native mystical and magickal Tibetan practices.
Sources for this write-up: www.advaita-vedanta.org/, www.sentient.org/amber/advaita.htm