a meson having a positive or negative charge and a rest mass 273.13 times that of the electron, or no charge and a rest mass 264.14 times that of the electron
Word origin
C20: from Greek letter pi1 + on
pion in American English
(ˈpaɪˌɑn)
noun
Particle Physics
any of three short-lived mesons that may be positive, negative, or neutral: pions have a mass c. 270 times that of an electron and play an important role in the binding forces within the nucleus of an atom