The American Heritage® Dictionary of the English Language, 4th Edition
n. A hypothetical form of matter that is identical to physical matter except that its atoms are composed of antielectrons, antiprotons, and antineutrons.
n. Matter that is composed of the antiparticles of those that constitute normal matter.
n. A form of matter that has a key property, such as charge, opposite to that of ordinary matter.
the GNU version of the Collaborative International Dictionary of English
n. Matter whch is composed of antiparticles such as antiprotons, positrons, and antineutrons.
WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
n. matter consisting of elementary particles that are the antiparticles of those making up normal substances
Word Usage
"Kobayashi and Maskawa hypothesized the existence of a third family of quarks, which are some of the building blocks from which all matter and antimatter is formed."