pion /pī″on/ [Gk, pi, 16th letter of Greek alphabet, meson, nuclear particle] , any of a family of subatomic particles that can be created in nuclear reactions. Pions are unstable but can exist long enough to be formed into beams and used in certain types of medical therapy, such as the treatment of brain tumors. Pions of suitable energy can penetrate the skull and deliver most of their energy to a tumor while sparing overlying normal tissue. See also negative pi meson pion.
