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
Any of the three least massive mesons having a positive, neutral, or negative electric charge. The charged pions have a mass 273 times that of an electron and a mean lifetime of 2.6 × 10-8 second and the neutral pion has a mass 264 times that of an electron and a mean lifetime of 8.4 × 10-17 second. Also called pi meson. See Table at subatomic particle.