eccentricity

Etymology

It is formed from English eccentric + -ity,^[2][3][4] or else borrowed from Medieval Latin eccentricitās, formed from eccentricus and the suffix -itās.^[5][6][4] Eccentric is from Late Latin eccentricus, from Ancient Greek ἔκκεντρος (ékkentros, “out of the centre”), as opposed to concentric, from ἐκ (ek, “out of”) and κέντρον (kéntron, “centre”).^[2][7][8]

Earliest recorded meaning

Of planetary orbits, the fact of not having the Earth exactly at their centre: 1556, in Robert Recorde's Castle of Knowledge.^[2] Merriam-Webster Unabridged dates the word to 1545,^[5] and Etymonline to the 1540s.^[4]

Semantic evolution

The word began as a term of Ptolemaic astronomy, for the quality of being off-centre or not concentric.^[2][4] From 1696 it was used of a curve's deviation from a circle, and since 1715 as a measurable quantity, as in the eccentricity of an ellipse or an orbit.^[2] Johnson (1755) gave the astronomical senses, and also "excursion from the proper orb", quoting Wotton on a duke's eccentricity abroad.^[9] The figurative sense of people came in two steps: an instance of odd or extravagant behaviour is recorded from 1657, and the quality or habit of deviating from what is usual, oddity, from 1794.^[2][4]

Timeline

• 1556 – of planetary orbits, the quality of not being centred on the Earth; of not being concentric.^[2] Merriam-Webster Unabridged dates the word to 1545.^[5]
• 1657 – an instance of odd or extravagant behaviour; an eccentricity.^[2][4]
• 1696 – of a curve, deviation from circular form.^[2]
• 1715 – this deviation as a measurable quantity.^[2]
• 1794 – the habit of deviating from what is usual; oddity, whimsicality.^[2][4]
• 1825 – distance from the centre (obsolete by 1856).^[2]

Meanings

Noun

1. The quality of being eccentric; odd or unconventional behaviour: Her eccentricity charmed the neighbours.^[10][5][2]
2. An odd habit or peculiarity: One of his eccentricities was wearing two watches.^[6][2]
3. Deviation of a path or orbit from a circle.^[10][2]
4. (mathematics) A number expressing the shape of a conic section, the ratio of a point's distance from the focus to its distance from the directrix; for an ellipse, the distance between the foci divided by the length of the major axis.^[10][3]
5. (machinery) The distance between the centre of a body and the axis about which it turns.^[5][6]

Related words

• eccentric
• concentric
• center

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 Oxford English Dictionary Online (accessed March 2026), Oxford: Oxford University Press
3. ^ a b Shorter Oxford English Dictionary on Historical Principles (2002), 5th ed., ed. William R. Trumble and Angus Stevenson, Oxford: Oxford University Press
4. ^ a b c d e f g Harper, Douglas. Online Etymology Dictionary, https://www.etymonline.com
5. ^ a b c d e Merriam-Webster Unabridged Dictionary (2024), online ed., Springfield, Mass.: Merriam-Webster
6. ^ a b c Dictionary.com Unabridged (2016), based on the Random House Unabridged Dictionary, New York: Random House
7. ^ The American Heritage Dictionary of the English Language (online 2024), 5th ed., Boston: Houghton Mifflin Harcourt
8. ^ Klein, Ernest. A Comprehensive Etymological Dictionary of the English Language. 2 vols. Amsterdam: Elsevier, 1966–1967.
9. ^ Johnson, Samuel (1755), A Dictionary of the English Language, London; 4th ed. 1773; Johnson's Dictionary Online, ed. Beth Rapp Young et al.
10. ^ a b c Collins English Dictionary (2023), Glasgow: HarperCollins