Nonduality points to something beyond words and is impossible to define precisely. If you look at a dictionary definition of nonduality, you will find something like this: "the implication that things appear distinct while not being separate" or "the indivisibility of reality."
Some describe nonduality as Oneness. Others use phrases such as "Consciousness is all" or "Everything is Consciousness." Ramana Maharshi once said "there is a single immanent reality, directly experienced by everyone, which is simultaneously the source, the substance and the real nature of everything that exists."
If you're at the beginning stages of nondual inquiry, don't worry about getting a firm hold on a definition. I'm just attempting to give you a flavor of what nondual inquiry and nondual teachings are all about. Here are three insightful discussions of nonduality that you might find helpful:
At an early stage of your interest in nonduality, you will likely be led to investigate the nature of the personal identity. In other words, you'll ponder the classic question, "Who am I?"
Most people who come to this inquiry have the conditioned belief, shared by the vast vast majority of the population, that what they are is a personal, limited consciousness located inside the skin line of the body. When you search for this personal "ME" inside the body, you won't find it.
If you remain open, the nondual investigation will reveal that what you are is the Awareness (sometimes referred to as Consciousness or Presence) in which all phenomena (including your body, thoughts, feelings, sensations and perceptions) appear.
You are not a thing. However, you ARE the source and substance of everything that appears. Don't even try to figure this out mentally right now. It won't work. Your mind will never grasp this.
This isn't easy for most people to swallow right away. But if you stick with the nondual investigation, you'll begin to see yourself (and others) very differently. And you will see the absurdity of many of the beliefs you now hold. You will clearly see them as false.
This isn't about adopting a new set of beliefs. You can't believe your way into nonduality -- although your mind will attempt hundreds of times to do just that! Effective nondual inquiry requires a willingness to investigate your current beliefs and to admit how much you don't know.
You're going to make some fascinating discoveries as you investigate your true nature. But don't be too quick to share what you're learning with friends and family. They'll think you're crazy!