up·​si·​lon | \ˈüp-sə-ˌlän, ˈyüp-, ˈəp-, -lən, British usually yüp-ˈsī-lən \

Definition of upsilon 

1 : the 20th letter of the Greek alphabet — see Alphabet Table

2 : 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

First Known Use of upsilon

1621, in the meaning defined at sense 1

History and Etymology for upsilon

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

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The first known use of upsilon was in 1621

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