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noun 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

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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

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