Advaita Vedanta (IAST Advaita Vedānta; Sanskrit: अद्वैत वेदान्त [əd̪ʋait̪ə ʋeːd̪ɑːnt̪ə]) is considered to be the most influential and most dominant sub-school of the Vedānta (literally, end or the goal of the Vedas, Sanskrit) school of Hindu philosophy. Other major sub-schools of Vedānta are Viśishṭādvaita and Dvaita; while the minor ones include Suddhadvaita, Dvaitadvaita and Achintya Bhedabheda. Advaita (literally, non-duality) is a system of thought where "Advaita" refers to the identity of the Self (Atman) and the Whole (Brahman).
The key source texts for all schools of Vedānta are the Prasthanatrayi—the canonical texts consisting of the Upanishads, the Bhagavad Gita and the Brahma Sutras. The first person to explicitly consolidate the principles of Advaita Vedanta was Shankara Bhagavadpada, while the first historical proponent was Gaudapada, the guru of Shankara's guru Govinda Bhagavatpada.
According to Śankara and others, anyone seeking to follow the philosophy of Advaita Vedānta must do so under the guidance of a Guru (teacher). The Guru must have the following qualities (see Mundaka Upanishad 1.2.12):
Someone told me once
Nothing is in vain
Everybody's dreaming
And everything's the same
Someone told me once
The time is now
Forever is still
Just like the ocean
Vain dreaming, saying nothing, once somebody told
Vain dreaming, saying nothing, once somebody told
Vain dreaming, saying nothing, once somebody told
Vain dreaming, saying nothing, once somebody told