antineutron
    n 1: the antiparticle of a neutron

antineutron \antineutron\ n.
   The antiparticle of a neutron. It has the same mass as a
   neutron, but a spin and magnetic moment opposite that of the
   neutron.
   [WordNet 1.5]

antipartticle \an`ti*part"ti*cle\
   ([a^]n`t[i^]*[aum]r"t[i^]*k'l), n. (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.
   [PJC]