<p><strong>Noun:</strong></p><ul><li>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: Compare antimatter, annihilation (def 3).</li><li>The positron is the antiparticle of the electron.</li></ul>