Nonduality can’t be defined in the same way you define … tungsten, for example. There’s no single definition of nonduality that everyone would agree upon. Nor is a definition of nonduality intended to replace full teachings such as Advaita Vedanta or Buddhist traditions, or the teaching of any Guru. Nonduality is to be seen, lived, awakened to, and then, for whatever reason, one may try to define it.
Having awakened to nonduality or the realization that there is only awareness, the elements of your definition may include a statement that nonduality means non-separation:
“In the world of things as they are, there is no self, no non self;” a confession from pure knowing, or experience, of what nonduality is:
“Nonduality is the living heart of being;” a method for experiencing nonduality:
“Don't keep searching for the truth; just let go of your opinions; a statement that the definition of nonduality rests in seeing or experiencing nonduality, not merely reading words:
“It is not simply having an occasional experience of unity beyond all division, it is actually being undivided. This is what nonduality truly means;”
a metaphor: “as the space in a jar delineates a part of main space: when the jar is broken, the individual space becomes once more part of the main space;”
a reference to an authority: “Gaudapada … argues that there is no duality;”
a disclaimer, paradox: “Nonduality is ineffable. Any words that attempt to capture its essence instead hide it from us.”
You’ll see several of those points in this description of nonduality or advaita from Encyclopedia Britannica:
Gaudapada … 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), in which individuals may be temporarily delineated just as the space in a jar delineates a part of main space: when the jar is broken, the individual space becomes once more part of the main space.
Almost all of the elements are found in this experiential revelation of nonduality (which literally means “not two”), composed by Seng T'san, the third Zen Patriarch, in The Mind of Absolute Trust:
Don't keep searching for the truth; just let go of your opinions.
For the mind in harmony with the Tao, all selfishness disappears.
With not even a trace of self-doubt, you can trust the universe completely.
All at once you are free, with nothing left to hold on to.
All is empty, brilliant, perfect in its own being.
In the world of things as they are, there is no self, no non self.
If you want to describe its essence, the best you can say is "Not-two."
In this "Not-two" nothing is separate, and nothing in the world is excluded.
The enlightened of all times and places have entered into this truth.
In it there is no gain or loss; one instant is ten thousand years.
There is no here, no there; infinity is right before your eyes.
The tiny is as large as the vast when objective boundaries have vanished;
the vast is as small as the tiny when you don't have external limits.
Being is an aspect of non-being; non-being is no different from being.
Until you understand this truth, you won't see anything clearly.
Most definitions of nonduality or advaita are found in the words of teachers.
When you go beyond awareness, there is a state of non-duality, in which there is no cognition, only pure being. In the state of non-duality, all separation ceases.
Nonduality is the living understanding of one's true nature as that which is empty of itself and full of the unfolding of being. Nonduality is a wave as the living relationship of complementary extremes. Nonduality, like Emptiness or Silence, is not a thing yet no thing can be without it. Nonduality is the Numberless One. Nonduality is the living heart of being. Nonduality is its own knowing as "Form is Seeing and Seeing is Being" (Quote is from Atmananda Krishna Menon). Nonduality is the path that is formed by walking it.
Some define nonduality at length. The following is part of a discussion by philosopher James M. Corrigan:
Nonduality is the state or condition of not being separate and distinct even if appearing to be so. It is the condition which allows us to say that there is no true separation between ourselves and anyone else or anything else in the world, for instance. When we say things like “We are all one,” or “God is in all things,” we are asserting that the presence we call ‘reality’ is a nonduality.
A “nondualism” is a systematic description of nondual reality, or the tradition of spiritual practices of nonduality.
More Complicated Answer:
Nonduality is the condition that one arrives at when all distinctions and relations between ‘things’ are removed. Fundamentally, all such distinctions and relations are the result of error on our part because it is we that impose the idea of plurality on the whole. Nonduality is thus a simple wholeness, rather than an “all in one” whole. It is very difficult to clearly contemplate such a simple wholeness, because by thinking about it and conceptualizing it, we have lost the simple wholeness that is the real nonduality that we were trying to grasp, which is always already our very nature – we are reality. Nonduality is the Infinite because we can both indivisibly apprehend it and enumerate it inexhaustibly into parts and relations between parts.
Nondualism according to this understanding is an error because fundamentally anything that we say or think about the Infinite removes this simple wholeness of reality from our grasp.
The Ultimate Answer:
Nonduality is ineffable. Any words that attempt to capture its essence instead hide it from us
Nondualism according to this understanding must be an apophatic* performance that uses words to lead us towards nonduality and then at the horizon of understanding pulls these words away so that reality can stand in its pure simple beauty. For example, in order to say that Nonduality is ineffable, we first posit 'Nonduality', making ‘Nonduality’ a creature of reason and thus positively identifying ‘Nonduality’ as some thing that can be thought about, and then in the same breath we take away this assertion by adding that this ‘what’ of which we speak is ineffable and thus beyond the reach of reason. The point being made by this performance is that Nonduality is not nothing, because then we could not even speak of ‘it’; but it is not something either, because if it were it could not be Nonduality; yet it is all things and no thing itself. Thus the name "Nonduality" is used to indicate a denial of multiplicity, yet the mind, seeing this denial, may assume that it means 'One' as that is the opposite of multiplicity in quantitative reasoning, and while reasoning the mind is locked into certain forms of thought, among them the form of contradictories. But the name "Nonduality," while it denies multiplicity, also denies its contradiction and subsumes both. These words are an apophatic performance. If you can ‘see’ their meaning, you do not need any more definitions.
There are many more definitions of nonduality, short and long, at http://nonduality.com/whatis.htm
For an extensive, in-depth comparative study, read Nonduality: A Study in Comparative Philosophy, by David Loy:
For a classic and simply told explanation of nondual reality, read Chuck Hillig’s Enlightenment for Beginners.
The Book: On the Taboo Against Knowing Who You Are, by Alan Watts is a classic from the Sixties. His use of the word nonduality may be the first in popular spiritual literature in the West.
The book I edited, One: Essential Writings on Nonduality, is filled it with diverse writings from nondual traditions and nondual perspectives. There’s something for everyone.
The word nonduality
One of my stumblings into nonduality was upon the word itself. One day in the early 80s I was sitting at the counter of a deli in Santa Monica and the guy beside me was reading an oversized leather bound book. I casually asked him what he was reading. He said, “The Upanishads.” I asked him what it was about. He didn’t answer me at first. He looked away, up and down, like he didn’t want to be bothered with me, but also as though he felt obligated to say something. After a few seconds he looked straight into my eyes and said the word as though it were a challenge: “Non-du-al-i-ty.”
In that way the unspoken power of the word was given to me. Here are 7 reasons why the word nonduality is powerful:
1. The word nonduality is fresh and untainted by loose-end connotations, such as the words Zen, consciousness, or spirituality are.
2. The word nonduality, for full understanding, demands its experience.
3. The word is a key that opens search engines and allows you to encounter, engage, and contribute to a multitude of teachings, conversations, and people.
4. The word nonduality is a portal, revealing the nondual perspective of many fields of knowledge: literature, psychology, cinema, education, art, physics, neurosciences, ecology, philosophy, mathematics, architecture, dance, music, martial arts, and more.
5. The word, like any word when it is first heard and valued, is a magnet. Once a person hears the word nonduality, it becomes a magnet drawing attention to other appearances of the word. As well, the user of the word becomes a magnet for others who are sensitive to the appearance of the word nonduality.
6. The word nonduality is a “red pill.” Recall that in the movie The Matrix, Neo was offered either a blue pill or a red pill. The blue pill would have returned Neo back to his dream world whose unreality he sensed but did not understand. The red pill would have awakened him to who he really was, which would have begun his journey through life and to the source.
The word nonduality could work as a red pill if you value its meaning enough to follow it as deeply as you can. When Neo was being given his choice of pills, his teacher and mentor Morpheus explained to him:
This is your last chance. After this there is no turning back. You take the blue pill, the story ends, you wake up in your bed and believe whatever you want to believe. You take the red pill, you stay in Wonderland, and I show you how deep the rabbit hole goes…. Remember, all I’m offering is the truth, nothing more….
7. The word is a pointing to “the seemingly indefinable underlying nature of reality.”
8. The word is a category holding “the body of works created by those who are open to experiencing life/reality beyond the limiting beliefs and definitions of mind.”
At one time the word nonduality was essentially unknown to the public. The word belonged to philosophers, scholars in comparative religion, Buddhists, and Hindus. The last ten years have seen the coming together of world teachers and the Internet, the wide dissemination of the teaching of nonduality in a variety of forms, the waking up of many, and the entry of the word nonduality into the spirituality mainstream.
A person at any level of understanding should be able to find a definition or description of nonduality that makes sense. However, I haven’t included the most elementary definitions of nonduality, for example, as one young person explained it to her not-so-spiritually-inclined parents, “Beyond good and bad.”
The collection of material on the definition of nonduality is still being generated, identified, accumulated, and published. Right now the collection is diverse, accessible, and open to new offerings and commentaries.