n.1.(Physics) A fundamental particle which has the same mass as one of the common fundamental particles, but which has an opposite charge, and for which certain other of the properties (e. g. baryon number, strangeness) may be opposite to that of the normal particle. The antiparticle to an electron is called a positron; the antiparticle to a proton is called an antiproton; the antiparticle to a neutron is called an antineutron. When a particle and its corresponding antiparticle collide, they typically annihilate each other with the production of large quantities of energy, usually in the form of radiation. The interaction of a proton and antiproton cause annihilation with production of mesons.
Webster's Revised Unabridged Dictionary, published 1913 by G. & C. Merriam Co.