opacity

Etymology

It is borrowed from Middle French opacité,^{[2][3][4][5][6][7][8]} from Latin opācitās (“shade, shadiness”),^{[2][3][6][5][9]} from opācus (“shaded, dark”),^[2][3][6][5] the source of opaque.^[6][9] Etymonline calls the Latin adjective a word of unknown origin.^[3] The OED dates the French word to 1512 in the sense of shadow, 1667 for lack of transparency and 1690 for obscurity of mind, and notes that in the Latin of British sources it meant obscurity from about 1150 and lack of transparency from about 1360.^[2]

Earliest recorded meaning

Obscurity of meaning: 1575, in J. Rolland's Treatise of the Court of Venus.^[2][5] Etymonline dates this sense to the 1550s and Barnhart to 1560, both as darkness or obscurity of meaning.^[3][6]

Semantic evolution

The first English sense was figurative, the obscurity of a text or idea; the OED notes that it was rare before the 20th century.^[2] The literal sense of being shaded, of darkness or dimness, followed in 1611, in Cotgrave's dictionary,^[2] and the physical sense of not letting light through in 1634,^[2] in the 1630s according to Etymonline.^[3] Johnson (1755) defined the word as cloudiness, a want of transparency.^[10] In the 19th century it came to be used of an opaque patch in something otherwise clear (1796), of slowness of mind (1806) and of resistance to other radiation, such as sound, heat or X-rays (1879).^[2] In the 20th century philosophers and linguists gave it technical senses: from 1956 the property of a context in which replacing a term by an equivalent one can change the truth of a statement, and since 1971 the property of a phonological rule that cannot be read off every case.^[2][11]

Timeline

• 1575 – obscurity of meaning.^[2][5]
• 1611 – the state of being in shadow; darkness.^[2]
• 1634 – the quality of not letting light through.^[2][3]
• 1796 – an opaque area in something transparent.^[2]
• 1806 – dullness of mind.^[2]
• 1879 – resistance to radiation other than light.^[2]
• 1956 – in philosophy and linguistics, the property of an opaque context.^[2]

Meanings

Noun

1. The quality or state of being opaque; lack of transparency or translucency.^[11][5][2]
2. The degree to which something is opaque, as a measure: The paint's opacity let one coat cover the old colour.^[11][5]
3. (physics, photography) The ratio of the intensity of light falling on a medium, such as a film, to that passing through it.^[11][2]
4. An opaque area, object or substance.^[11][2]
5. Obscurity of meaning; difficulty of understanding: the opacity of the committee's report.^[11][5][2]
6. (logic, philosophy) The property of being an opaque context.^[11][2]
7. Darkness or shade; the state of being in shadow.^[5][2]
8. (rare) Dullness of mind.^[2][7]

Word family

• nonopacity

Related words

• opaque
• opacify

References

1. ^ Wiktionary, the free dictionary, https://en.wiktionary.org
2. ^ a b c d e f g h i j k l m n o p q r s t u v w x Oxford English Dictionary Online (accessed March 2026), Oxford: Oxford University Press
3. ^ a b c d e f g Harper, Douglas. Online Etymology Dictionary, https://www.etymonline.com
4. ^ The American Heritage Dictionary of the English Language (online 2024), 5th ed., Boston: Houghton Mifflin Harcourt
5. ^ a b c d e f g h i Merriam-Webster Unabridged Dictionary (2024), online ed., Springfield, Mass.: Merriam-Webster
6. ^ a b c d e Barnhart, Robert K. (1988), The Barnhart Dictionary of Etymology
7. ^ a b Shorter Oxford English Dictionary on Historical Principles (2002), 5th ed., ed. William R. Trumble and Angus Stevenson, Oxford: Oxford University Press
8. ^ Stevenson, Angus, ed. (2010), Oxford Dictionary of English, 3rd ed., Oxford: Oxford University Press
9. ^ a b Klein, Ernest. A Comprehensive Etymological Dictionary of the English Language. 2 vols. Amsterdam: Elsevier, 1966–1967.
10. ^ Johnson, Samuel (1755), A Dictionary of the English Language, London; 4th ed. 1773; Johnson's Dictionary Online, ed. Beth Rapp Young et al.
11. ^ a b c d e f g Collins English Dictionary (2023), Glasgow: HarperCollins