The Collaborative International Dictionary of English v.0.48:

antimatter \an`ti*mat"ter\ ([a^]n`t[i^]*m[a^]t"t[~e]r), n.
   (Physics)
   Matter whch is composed of antiparticles such as antiprotons,
   positrons, and antineutrons.
   [PJC]

WordNet (r) 3.0 (2006):

antimatter
    n 1: matter consisting of elementary particles that are the
         antiparticles of those making up normal substances