duality

Etymology

It is inherited from Middle English dualite,^[2][3] borrowed from Middle French dualité,^[4][5][6] from Late Latin duālitās (“the number two; twofold nature”),^[5][7][6][8] from duālis (“containing two; dual”),^[5][7][8] which comes from duo (“two”), from Proto-Indo-European *dwo- (“two”).^[7] The OED dates the French word to the 14th century,^[5] while the MED, which finds French dualité only in the 16th century, only compares it and the Latin noun,^[2] and Etymonline and the Oxford Dictionary of English take the English word straight from Late Latin.^[7][9] The suffix is -ity.^[6][3]

Earliest recorded meaning

Twofold nature, the condition of being two: in Usk's Testament of Love, written about 1385 but known only from the printed edition of 1532,^[5][2] which says that this dualite is found in every creature.^[2] Etymonline dates the word to the late 14th century,^[7] Dictionary.com to 1350–1400.^[3]

Semantic evolution

In Middle English the word also meant simply the number two: a treatise on music from before 1450 says that four is divided into two dualiteis.^[2] The general sense of being twofold or made up of two parts has stayed the main one.^[5][4] In the 17th century it was briefly used in church law for the holding of two benefices at once, a sense the Shorter Oxford explains as modelled on plurality;^[5][8] it is recorded from 1620 to 1663.^[5] In physics the word names the principle that light and matter behave both as waves and as particles, the wave-particle duality.^[10]

Timeline

• c. 1385 – twofold nature, in Usk's Testament of Love (printed 1532).^[5][2]
• before 1450 – the number two.^[2]
• 1620 – the holding of two benefices together (obsolete by 1663).^[5]

Meanings

Noun

1. The quality or state of being dual or made up of two parts or aspects; doubleness or a dichotomy: the duality of body and soul.^{[4][11][10][5]}
2. (physics) The principle that electromagnetic radiation and particles show both wave and particle properties, so that wave theory and corpuscular theory complement each other.^[10]
3. (obsolete, 1620–1663) The holding of two benefices at the same time.^[5][8]

Word family

• nonduality
• wave-particle duality

Related words

• dual
• dualism
• duo
• two
• plurality

References

1. ^ Wiktionary, the free dictionary, https://en.wiktionary.org
2. ^ a b c d e f g Kurath, Hans; Kuhn, Sherman M.; Lewis, Robert E., eds. (1952–2001), Middle English Dictionary, Ann Arbor: University of Michigan Press; online edition ed. Frances McSparran et al. (2000–2018)
3. ^ a b c Dictionary.com Unabridged (2016), based on the Random House Unabridged Dictionary, New York: Random House
4. ^ a b c Merriam-Webster Unabridged Dictionary (2024), online ed., Springfield, Mass.: Merriam-Webster
5. ^ a b c d e f g h i j k l Oxford English Dictionary Online (accessed March 2026), Oxford: Oxford University Press
6. ^ a b c Klein, Ernest. A Comprehensive Etymological Dictionary of the English Language. 2 vols. Amsterdam: Elsevier, 1966–1967.
7. ^ a b c d e Harper, Douglas. Online Etymology Dictionary, https://www.etymonline.com
8. ^ a b c d Shorter Oxford English Dictionary on Historical Principles (2002), 5th ed., ed. William R. Trumble and Angus Stevenson, Oxford: Oxford University Press
9. ^ Stevenson, Angus, ed. (2010), Oxford Dictionary of English, 3rd ed., Oxford: Oxford University Press
10. ^ a b c Collins English Dictionary (2023), Glasgow: HarperCollins
11. ^ The American Heritage Dictionary of the English Language (online 2024), 5th ed., Boston: Houghton Mifflin Harcourt