Pronunciation: (an'tē-pär"ti-kul, an'tī-),[key] —n.Physics. a particle all of whose properties, as mass, spin, or charge, have the same magnitude as but, where appropriate, the opposite sign of a specific elementary particle; neutral pions, photons, and gravitons are considered to be their own antiparticles: The positron is the antiparticle of the electron. Cf. antimatter, annihilation (def. 3).