Mystical Union, Nonduality
Variety of expressions in mystical unity, nonduality
Advaita

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 Encyclopędia 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. (Encyclopędia Britannica).
Unity of Being

       The most famous idea attributed to Ebn al-'Arabi is wahdat al-wojud "the oneness
       of being." Although he never employs the term, the idea is implicit throughout his
       writings. In the manner of both theologians and philosophers, Ebn al-'Arabi
       employs the term wojud to refer to God as the Necessary Being. Like them, he also
       attributes the term to everything other than God, but he insists that wojud does not
       belong to the things found in the cosmos in any real sense. Rather, the things
       borrow wojud from God, much as the earth borrows light from the sun. The issue
       is how wojud can rightfully be attributed to the things, also called "entities"
       (a'yan). From the perspective of tanzih, Ebn al-'Arabi declares that wojud
       belongs to God alone, and, in his famous phrase, the things "have never smelt a
       whiff of wojud." From the point of view of tashbih, he affirms that all things are
       wojud's self-disclosure (tajalli) or self-manifestation (zohur). In sum, all things
       are "He/not He" (howa la howa), which is to say that they are both God and other
       than God, both wojud and other than wojud. 

       The intermediateness of everything that can be perceived by the senses or the mind
       brings us back to imagination, a term that Ebn al-'Arabi applies not only to a mode
       of understanding that grasps identity rather than difference, but also to the World
       of Imagination, which is situated between the two fundamental worlds that make up
       the cosmosóthe world of spirits and the world of bodiesóand which brings
       together the qualities of the two sides. In addition, Ebn al-'Arabi refers to the whole
       cosmos as imagination, because it combines the attributes of wojud and utter
       nonexistence (Chittick, 1989).