Etymologically
Log in

tau

This is the current revision of this entry, as edited by bogdan on 31 October 2026 at 23:51. This address is a permanent link to this version.
Changes in this edit History

Pronunciation

  • UK: /taʊ/, /tɔː/[1]
  • US: /taʊ/, /tɔ/[1]

Etymology

It is inherited from Middle English tau, also taue and thau,[2] borrowed from Latin tau, in post-classical Latin also thau,[3][2] partly also borrowed from Old French thau.[2] The Latin word is the name of the letter T in the Greek alphabet, Ancient Greek ταῦ (taû),[3][4][5] which is of Semitic origin:[6][7][8] the OED traces it to the name of the letter in Phoenician,[3] and Klein and Etymonline to Biblical Hebrew תָּו (tāw, “sign, mark”), the name of the last letter of the Hebrew alphabet.[4][9][8] Etymonline dates the English word to the early 14th century, in the form taw.[9]

Earliest recorded meaning

A mark or figure in the shape of the letter T, especially as a Christian symbol of the Cross: already in Old English, in Ælfric's Catholic Homilies.[3] As the name of the Greek letter it is first recorded before 1400, in the Cursor Mundi.[3][2]

Semantic evolution

In post-classical Latin tau was used for the shape of Christ's cross from the late 2nd century, in Tertullian, and as the name of the last letter of the Hebrew alphabet from the 4th century.[3] Early English uses refer to marks in the Old Testament, such as the sign in Ezekiel 9:4, that were read as prefiguring the Cross;[3] in Middle English the word could mean the sign of the cross marked on the forehead in blessing or confirmation,[2][10] and a T-shaped cross, St Anthony's cross, became a tau cross.[9][10][5] From the 18th century it was also applied to the Egyptian ankh (1785).[3][9] The Shorter Oxford notes that tau was originally the last letter of the Greek alphabet.[10] Later uses build on the letter's shape or its place in the alphabet: the nineteenth star of a constellation (1786), insects marked with a T (1808) and a T-headed pastoral staff (1855).[3] In physics the name went first to the tau meson (1949), now identified with the positive kaon, and then to the tau lepton (1977);[3][10] according to Wiktionary, Martin Perl chose the letter for the lepton after Ancient Greek τρίτον (tríton, “third”), as it was the third charged lepton.[11] In biochemistry tau has named a protein of the nerve cells since 1975.[3][8]

Timeline

  • Old English – a T-shaped sign as a Christian symbol, in Ælfric.[3]
  • before 1400 – the name of the Greek letter T, in the Cursor Mundi.[3][2]
  • 1785 – the ankh.[3]
  • 1786 – in astronomy, the nineteenth star of a constellation.[3]
  • 1808 – in the names of insects with T-shaped markings.[3]
  • 1855 – a pastoral staff with a T-shaped head.[3]
  • 1949 – the tau meson, in particle physics.[3]
  • 1975 – a protein of the microtubules.[3]
  • 1977 – the tau lepton.[3]

Meanings

Noun

  1. The nineteenth letter of the Greek alphabet (Τ, τ), corresponding to T.[7][8][3]
  2. A T-shaped mark or object, such as a tau cross or St Anthony's cross, a T-headed crosier or an ankh.[8][3][10]
  3. (particle physics) A short-lived, heavy, negatively charged lepton, also called a tau particle or tauon.[8][3][10]
  4. (biochemistry) A protein that binds to and stabilizes the microtubules of nerve cells.[8][3]
  5. (astronomy) Used in the name of the nineteenth star of a constellation: Tau Ceti.[3]
  6. (historical) In physics, the tau meson, a particle now identified as a kaon.[3][10]

Word family

  • tauon
  • tau cross
  • tau lepton
  • tau neutrino
  • taw
  • T

References

  1. ^ a b Wiktionary, the free dictionary, https://en.wiktionary.org
  2. ^ a b c d e f 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 d e f g h i j k l m n o p q r s t u v w x y z Oxford English Dictionary Online (accessed March 2026), Oxford: Oxford University Press
  4. ^ a b Klein, Ernest. A Comprehensive Etymological Dictionary of the English Language. 2 vols. Amsterdam: Elsevier, 1966–1967.
  5. ^ a b Weekley, Ernest (1921), An Etymological Dictionary of Modern English, London: John Murray
  6. ^ The American Heritage Dictionary of the English Language (online 2024), 5th ed., Boston: Houghton Mifflin Harcourt
  7. ^ a b Collins English Dictionary (2023), Glasgow: HarperCollins
  8. ^ a b c d e f g Merriam-Webster Unabridged Dictionary (2024), online ed., Springfield, Mass.: Merriam-Webster
  9. ^ a b c d Harper, Douglas. Online Etymology Dictionary, https://www.etymonline.com
  10. ^ a b c d e f g Shorter Oxford English Dictionary on Historical Principles (2002), 5th ed., ed. William R. Trumble and Angus Stevenson, Oxford: Oxford University Press
  11. ^ Random House Webster's Unabridged Dictionary (1999), 2nd ed., New York: Random House