A pion (or pi-meson) is one of the particles that carry the strong nuclear force between protons and neutrons (and other hadrons), which binds an atomic nucleus together. Pions are denoted + , - , 0 , depending on charge.

Pions were predicted to exist in 1935 by the Japanese theoretical physicist Hideki Yukawa when, in the first paper he ever published, Hideki proposed there was a new force involved in holding the nucleus together. This new force was overcoming the positive charge on each proton that was trying to blow the nucleus apart. Eventually this force came to be known as the strong nuclear force. Yukawa predicted that this new force was carried by a massive particle with only a very short range (about 10 cm). Hideki predicted the mass of this new particle would be about 300 times that of the electron, or about 150 MeV. To account for all the possible interactions between nucleons...