Definition of upsilon in English:


Pronunciation /ˈʊpsɪlɒn//ʌpˈsʌɪlən//ˈjuːpsɪlɒn//juːpˈsʌɪlən/


  • 1The twentieth letter of the Greek alphabet (Υ, υ), transliterated as ‘u’ or (chiefly in English words derived through Latin) as ‘y’.

    • ‘Look lower down the alphabet - theta perhaps, or even upsilon.’
    • ‘Typesetters typically confuse Greek letters in setting Greek quotations: nu with upsilon, theta with phi.’
    1. 1.1Astronomy followed by Latin genitive The twentieth star in a constellation.
      ‘Upsilon Scorpii’
      • ‘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.’
      • ‘For example, Upsilon Andromedae is now known to possess a system of three large planets.’
      • ‘The discovery comes only seven months after the first multi-planet system outside our own, around the star Upsilon Andromedae, was identified.’
    2. 1.2Physics A meson thought to contain a b quark bound to its antiparticle, produced in particle accelerators.
      • ‘About eight times the mass of a proton, the zeta particle was created when electrons and positrons collided at about 10 Gev (gigavolts of energy), where it appeared among the decay products of the upsilon particle.’
      • ‘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.’
      • ‘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.’
      • ‘The new phenomena included major breakthroughs in physics, including the discovery of the upsilon particle, or bottom quark, in 1977.’
      • ‘The upsilon particle is a meson which was discovered at Fermilab in 1977.’
      • ‘The total energy of the muon pair corresponds to the mass of the upsilon particle, while the two jets appear to have come from the decay of a Z particle, carrier of the weak force.’


Greek, literally ‘slender U’, from psilos ‘slender’, referring to the need to distinguish upsilon from the diphthong oi: in late Greek the two had the same pronunciation.