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.
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.
A meson occurring either in a neutral form with a mass 264 times that of an electron and a mean lifetime of 8.4 × 10&spminus;17 seconds or in a positively or negatively charged form with a mass 273 times that of an electron and a mean lifetime of 2.6 × 10&spminus;8 seconds. The pion was once believed to be the particle that mediates the strong force , which holds nucleons together in the nucleus; it is now believed that the gluon is the mediator particle. Pions do interact with nucleons, however, and are able to transform neutrons into protons and vice versa.