antimatter[an′ti mat′ər; -tī-, -tē-]
a form of matter in which the electrical charge or other property of each constituent particle is the reverse of that in the usual matter of our universe: an atom of antimatter has a nucleus of antiprotons and antineutrons surrounded by positrons
A form of matter that is identical to ordinary matter except that it is composed of antielectrons, antiprotons, and antineutrons.
(usually uncountable, plural antimatters)
From anti- + matter.