up·​si·​lon ˈüp-sə-ˌlän How to pronounce upsilon (audio) ˈyüp- How to pronounce upsilon (audio)
 British usually  yüp-ˈsī-lən
: the 20th letter of the Greek alphabet see Alphabet Table
: any of a group of unstable electrically neutral elementary particles of the meson family that have a mass about 10 times that of a proton

Examples of upsilon in a Sentence

Recent Examples on the Web The team is hoping to spot composite particles called upsilons, for example. Clara Moskowitz, Scientific American, 14 Feb. 2023

These examples are programmatically compiled from various online sources to illustrate current usage of the word 'upsilon.' Any opinions expressed in the examples do not represent those of Merriam-Webster or its editors. Send us feedback about these examples.

Word History


Middle Greek y psilon, literally, simple y; from the desire to distinguish it from oi, which was pronounced the same in later Greek

First Known Use

1621, in the meaning defined at sense 1

Time Traveler
The first known use of upsilon was in 1621

Dictionary Entries Near upsilon

Cite this Entry

“Upsilon.” Dictionary, Merriam-Webster, Accessed 5 Oct. 2023.

Kids Definition


up·​si·​lon ˈyüp-sə-ˌlän How to pronounce upsilon (audio) ˈəp- How to pronounce upsilon (audio)
: the 20th letter of the Greek alphabet
Υ or υ
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