A nondual philosophical or religious perspective or theory maintains that there is no fundamental distinction between mind and matter. In Western philosophy, nondual views are often called monism. The term "nondual" is a literal translation of the Sanskrit term Advaita. Plotinus, Nagarjuna, Shankaracharya, F. H. Bradley, Aurobindo, Ramana Maharshi, Ram Dass, Ken Wilber, and Stuart Davis subscribe to nondual views of reality. Zen and Advaita Vedanta Hinduism are nondual views.

To the Nondualist, reality is ultimately neither physical nor mental. Instead, it is an ineffable state or realization. This ultimate thing can be called "Spirit" (Aurobindo), "Brahman" (Shankara), "God", "The One", "The All" (Plotinus), "The Self" (Ramana Maharshi), "The Absolute" (Schelling) or simply "The Nondual" (Bradley). Ram Dass calls it the "third plane"—any phrase will be insufficient, he maintains, so any phrase will do.

It must be pointed out that technically there can be no such thing as a nondual view or theory or experience, only a realization of Oneness or nonduality. That is to say, nonduality can only be achieved through a mystical union with all. Thus, technically, there cannot truly be a verbal account of this union, only words that insufficiently point to the realization.