An atom in which all the particles of an ordinary atom are replaced by their antiparticles, i.e. electrons by positrons, protons by antiprotons, and neutrons by antineutrons. An antiatom cannot co-exist with an ordinary atom since the atom and the antiatom would annihilate each other with the production of energy in the form of high-energy photons. Antiatoms have been created artificially in laboratories.