n.
Pronunciation: ' üp-s ə - ˌ län, ' yüp-, ' ə p-, -l ə n, Brit usu yüp- ' s ī -l ə n
Function: noun
Etymology: Middle Greek y psilon, literally, simple y; from the desire to distinguish it from oi, which was pronounced the same in later Greek
Date: 1621
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