Nonduality is a term originally derived from the Sanskrit word Advaita which means "not-two." What is not two is the apparent split between the perceiver and the perceived, subject and object, self and world. While these polarized distinctions appear to be obvious and are both deeply ingrained and quite compelling, they are fundamentally mental constructs. For millennia sages from a variety of wisdom traditions, both East and West, have penetrated these dichotomies and described an underlying unitive awareness that can be directly experienced.
Contact with this, either spontaneously or through practice, brings an awakening out of conventional dualistic identity into a non-state of spacious, open, lucid, radiantly empty awareness, which is often referred to as our true or essential nature. Once quite rare, these nondual "awakenings" are happening with increasing frequency and have significant implications for the field of psychology and psychotherapy.
(from Nondual Wisdom and Psychotherapy website)
All that is not One must ever
Suffer with the wound of Absence,
And whoever in Love's city
Enters, finds but room for One
And but in One-ness, Union.
(poet Jami on mystic union thru mystic love)