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
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Origin and Etymology of upsilon
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
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