antiparticle

n. one of two particles that destroy each other on impact (Physics)

Corresponding to most kinds of particle, there is an associated antiparticle with the same mass and opposite charges. (The exceptions are massless gauge bosons such as the photon.) Even electrically neutral particles, such as the neutron, are not identical to their antiparticle. In the example of the neutron, the 'ordinary' particle is made out of quarks and the antiparticle out of antiquarks. The laws of nature were thought to be symmetric between particles and antiparticles until CP violation experiments found that time-reversal symmetry is violated in nature. This small asymmetry is involved in baryogenesis, the process by which our universe came to consist almost entirely of matter, with almost no free antimatter.
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Noun
1. a particle that has the same mass as another particle but has opposite values for its other properties; interaction of a particle and its antiparticle results in annihilation and the production of radiant energy
(hypernym) elementary particle, fundamental particle, subatomic particle
(substance-holonym) antimatter