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1.
Not two. Non-separateness. The nature of reality.

The experience, sense, intuition, or knowing of a reality that is deeper, more meaningful, more unified, more vast than the reality that is known on a day to day basis, and from which there is no separation.

The recognition that while non-separateness describes reality, distinct and apparently separate forms arise within it. The further recognition that form and reality are not separate even though they may appear to be, even as waves are not separate from the ocean.

The body of practices, teachings, teachers, traditions that may be accessed in the pursuit of understanding reality.

Beyond that, the perfect knowing of reality, such that there is not even a knower.
"I do not believe…that an individual may gain spiritually and those who around him suffer. I believe in ... nonduality, I believe in the essential unity of man and, for that matter, of all that lives. Therefore I believe that if one man gains spiritually, the whole world gains with him and, if one man falls, the whole world falls to that extent." Gandhi
by Jerry Katz October 27, 2007