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Definitions from web:

antiparticle (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)

Corresponding to most kinds of particles, there is an associated antiparticle with the same mass and opposite electric charge. For example, the antiparticle of the electron is the positively charged antielectron, or positron, which is produced naturally in certain types of radioactive decay.

A particle and its antiparticle have opposite values for all other non-zero quantum number labels. These labels are electric charge, color charge, flavor, electron number, muon number, tau number, and baryon number.

An antimatter counterpart of an elementary particle. Antiparticles are denoted my placing a bar above the symbol for a given particle. For example, the proton is denoted p, so the antiproton is denoted p’ (p-bar). An exception to this rule is that the antiparticle of the electron (e-)  is called the positron and denoted (e+).

sources:

 http://www2.slac.stanford.edu/vvc/theory/antiquarks.html

http://scienceworld.wolfram.com/physics/Antiparticle.html

http://en.wikipedia.org/wiki/Antiparticle