Advaita Vedanta...

Introduction: The fundamental knowledge is Atman is Brahman. The Atman being the "Self" and Brahman means the "All Soul" or the Universal Consciousness. Vedas speak of mystical union as the realization that Atman is Brahman.

Advaita is a Sanskrit word that literally means "not two". Modern interpretation of Advaita is sometimes presented as "Nonduality" and even revised as the end of the Vedas or "Nonduality beyond knowledge." Another name for the study of Advaita is Jnani (knowledge) Yoga. In the 20th century, modern Advaita masters Ramana Maharshi, Nisargadatta Maharaj broke away from the traditional, scripture driven path and they spoke directly from their experience.

Among the variety of reinterpretations, to find the most common understanding, I searched Encyclopædia Britannica:

7th-century thinker Gaudapada, author of the
Mandukya-karika, argues that there is no duality; the
mind, awake or dreaming, moves through maya
("illusion"); and only nonduality (advaita) is the
final truth. This truth is concealed by the ignorance
of illusion. There is no becoming, either of a thing
by itself or of a thing out of some other thing. There
is ultimately no individual self or soul (jiva), only
the atman (all-soul).

The medieval Indian philosopher Sankara, (700?-750?),
builds further on Gaudapada's foundation, principally
in his commentary on the Vedanta-sutras, the
Sari-raka-mimamsa-bhasya ("Commentary on the Study of
the Self ").

Sankara in his philosophy does not start
from the empirical world with logical analysis but,
rather, directly from the absolute (Brahman). If
interpreted correctly, he argues, the Upanisads teach
the nature of Brahman. In making this argument, he
develops a complete epistemology to account for the
human error in taking the phenomenal world for real.

Fundamental for Sankara is the tenet that the Brahman
is real and the world is unreal. Any change, duality,
or plurality is an illusion. The self is nothing but
Brahman. Insight into this identity results in
spiritual release. Brahman is outside time, space, and
causality, which are simply forms of empirical
experience. No distinction in Brahman or from Brahman
is possible.


(Encyclopædia Britannica).