In plain words
Nonduality is the teaching that reality is not finally split in two. The self and the world, the knower and the known, the human and the divine look like separate things, but at the deepest level, these traditions say, they are one reality seen from different sides. The English word comes from the Sanskrit advaita, "not two." Most teachers present nonduality as something recognized directly rather than a belief to be argued, and many add that it does not erase ordinary differences or moral responsibility. The traditions below name it in their own words and do not all mean exactly the same thing.
Across the traditions
Advaita Vedanta
In Hinduism, the school of Advaita Vedanta gave the idea its name. Advaita means "not two": atman, the deepest self, and Brahman, ultimate reality, are one, and the sense of being a separate self is a kind of mistake. Its best-known teacher is Adi Shankara, who lived around the 8th century CE. The Ashtavakra Gita, a Sanskrit dialogue of unknown date in the same current, puts it in an image:
"Waves, foam, and bubbles do not differ from water. In the same way, all this which has emanated from oneself is no other than oneself." —Ashtavakra Gita 2.4, trans. John Richards
Chan and Zen
In Chan and Zen Buddhism the phrase is "not two" (Chinese bu'er, Japanese funi): awakening and everyday life, the pure and the defiled, are not separate realms. Huineng, the Sixth Patriarch of Chan, calls the Buddha's teaching nondual in the Platform Sutra and treats awakening as a single moment of seeing rather than a ladder to climb.
"Good friends, ordinary people are buddhas, and the afflictions are bodhi." —Huineng, Platform Sutra, ch. 2, trans. John R. McRae (2000)
Taoism
In Taoism the one reality is the Tao, "the Way," which comes before every name. Names sort the world into pairs; the Tao is prior to the sorting. The Tao Te Ching, traditionally credited to Laozi, opens with exactly this point, and the same chapter goes on to call the nameless and the named one reality under two aspects.
"The Tao that can be trodden is not the enduring and unchanging Tao. The name that can be named is not the enduring and unchanging name." —Tao Te Ching, ch. 1, trans. James Legge (1891)
Sufism
In Islam, tawhid, the oneness of God, is the center of belief. Sufis also read it inwardly: in fana, "passing away," the seeker's sense of a separate self falls away until only God remains. Later Sufi philosophy called the idea wahdat al-wujud, "oneness of being." Al-Ghazali, the Persian theologian (1058–1111), set two levels of the same confession side by side:
"'There is no deity but ALLAH' is the Many's declaration of Unity: that of the Few is 'There is no he but HE.'" —Al-Ghazali, The Niche of Lights, Part I, trans. W. H. T. Gairdner (1924)
He was careful to add that the state is called "Identity" only in the language of metaphor; in the language of reality it is "Unification."
Christian mysticism
In Christianity the usual word is union: the soul made one with God through love and prayer, while most Christian writers hold that creature and Creator remain distinct. Julian of Norwich, the 14th-century English anchoress, used the Middle English verb "one," to make one, for it:
"Prayer oneth the soul to God." —Julian of Norwich, Revelations of Divine Love, ch. 43, ed. Grace Warrack
