Nondualism, also called nonduality and sometimes monism, is a polyvalent term originating in Indian philosophy and religion, where it is used in various, related contemplative philosophies which aim to negate dualistic thinking or conceptual proliferation (prapanca) and thereby realize nondual awareness, 'that which is beyond discursive thinking', a state of consciousness described in contemplative traditions as a background field of unified, immutable awareness that exists prior to conceptual thought.

The English term "nonduality" is derived from the Sanskrit Hindu term "advaita" (अद्वैत), "not-two" or "one without a second," meaning that only Brahman, 'the one', is ultimately real while 'the world', or the multiplicity of thought-constructs, 'the second', is not fully real; and from the Buddhist term advaya, which is also literally translated as "not two" and has various applications, including the Madhyamaka negation of thinking in opposites such as ordinary, conventional truth versus ultimate truth, and in Yogachara the deconstruction of the "apprehension of sensory objects as separate from the perceiving consciousness."

A perennialist view posits that nondual awareness, despite fundamental differences in the explanatory frameworks, is a common essence in various religious traditions. According to this view, nondual awareness is not only paradigmatic for Hindu advaita-traditions including Advaita Vedanta and Kashmir Shaivism, and Buddhist advaya-traditions including Yogachara, Madhyamaka, Zen and Dzogchen, but can also be found in Taoist philosophy, and in Western philosophy, Christian mysticism, and Sufism.

Nondualism is also used to refer to the satsang movement, also called neo-advaita, for which nonduality is a central tenet, emphasizing sudden awakening or insight. The term may also refer to monism and nonplurality, the idea of a unitive essence behind the multiplicity of distinct entities. Related definitions include interconnectedness interdependence, and holism or 'wholism', the idea that "all the things "in" the world are not really distinct from each other but together constitute some integral whole." Further definitions are the rejection of thinking in binary opposites such as the mind–body dualism, while "nondualism" is also used as a synonym for mysticism, mystical experience, and spirituality.

"Nondualism" and "nonduality" are the translation of the Sanskrit terms advaita and advaya (Tibetan: gNis-med, Chinese: pu-erh, Japanese: fu-ni).

"Advaita" (अद्वैत) is from Sanskrit roots a, not; dvaita, "customarily translated as dual." As Advaita, it is usually translated as "not-two" or "one without a second", and most commonly as "nondualism", "nonduality" or "nondual," invoking the notion of a dichotomy. Fabian Volker, following Paul Hacker explains that dvaita does not mean "duality," but "the state in which a second is present," the second here being synonymous with prapanca, "conceptual proliferation," and with jagat, "the world." Advaita thus means that only Brahman, 'the one', is ultimately real, while the phenomenal world, or the conceptual multiplicity, 'the second', is not fully real. The term thus does not emphasize two instances, but the notion that the second instance is not fully real, and advaita is better translated as "that which has no second beside it" instead of "nonduality," denying multiplicity and the proliferation of concepts "that tend to obscure the true state of affairs."

"Advaya" (अद्वय) is also a Sanskrit word that means "identity, unique, not two, without a second", and typically refers to sunyata ('emptiness') and the two truths doctrine of Mahayana Buddhism, especially Madhyamaka, and the negation of the conceptual duality between observer and observed in Yogacara. The term prapanca, conceptual proliferation and the creation of a multi-faceted world, is also used in these discourses. Hookham renders nisprapanca as "nonconceptual," explaining:

...terms such as Appearance-Emptiness, Bliss-Emptiness, Clarity-Emptiness, Awareness-Emptiness both-at-once (yuganaddha) are used to refer to the Ultimate (Paramartha) Nature-of-Mind. "Emptiness" in these combinations means nonceptual (nisprapanca) space or openness. In other words, though there is nothing to grasp (conceptually), Appearance, Bliss, Clarity or Awareness is still vividly present and is the Non-conceptual (Nis-prapanca) Reality.