Definition of upsilon in English:

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Pronunciation: /ˈ(y)o͝opsəˌlän/
Pronunciation: /ˈəpsəˌlän/


1The twentieth letter of the Greek alphabet (Υ, υ), transliterated in the traditional Latin style as ‘y’ (as in cycle) or in the modern style as ‘u’ (as in the etymologies of this dictionary).
Example sentences
  • Typesetters typically confuse Greek letters in setting Greek quotations: nu with upsilon, theta with phi.
  • Look lower down the alphabet - theta perhaps, or even upsilon.
1.1 (Upsilon) [followed by Latin genitive] Astronomy The twentieth star in a constellation: Upsilon Scorpii
More example sentences
  • The system they focused on, three huge Jupiter-like planets orbiting the central star Upsilon Andromedae, was the first extrasolar multi-planet system ever discovered by Doppler spectroscopy.
  • The discovery comes only seven months after the first multi-planet system outside our own, around the star Upsilon Andromedae, was identified.
  • For example, Upsilon Andromedae is now known to possess a system of three large planets.
1.2 (also upsilon particle) Physics A meson thought to contain a b quark bound to its antiparticle, produced in particle accelerators.
Example sentences
  • Use of the tracking information told the energies and momenta of the muons - which could then be traced back to find the new upsilon particle and its mass.
  • Similarly, the bottom quark was inferred from the discovery of the upsilon particle by Leon Lederman in 1977 at Fermilab.
  • Some colleagues were surprised that he had not won a Nobel earlier, perhaps for the discovery in 1977 of the upsilon particle, the most massive nuclear particle yet found.


Mid 17th century: Greek, literally 'plain or simple U', from psilos 'plain', referring to the need to distinguish upsilon from the diphthong oi: in late Greek the two had (and in modern Greek still have) the same pronunciation.

Words that rhyme with upsilon


For editors and proofreaders

Syllabification: up·si·lon

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