Main Entry: pi·on

Pronunciation: \ˈpī-ˌän\

Function: noun

Etymology: contraction of pi-meson

Date: 1950

: a meson that is a combination of up and down quarks and antiquarks, that may be positive, negative, or neutral, and that has a mass about 270 times that of the electron

— pi·on·ic \pī-ˈä-nik\ adjective