[an-tee-mat-er, an-tahy-]
noun Physics.
matter composed only of antiparticles, especially antiprotons, antineutrons, and positrons.

1950–55; anti- + matter Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
antimatter (ˈæntɪˌmætə)
a form of matter composed of antiparticles, such as antihydrogen, consisting of antiprotons and positrons

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
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Word Origin & History

also anti-matter, 1953, from anti- + matter.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Science Dictionary
antimatter   (ān'tĭ-māt'ər)  Pronunciation Key 
A form of matter that consists of antiparticles.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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American Heritage
Cultural Dictionary

antimatter definition

In physics, matter made of antiparticles.

The American Heritage® New Dictionary of Cultural Literacy, Third Edition
Copyright © 2005 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved.
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Example sentences
Antimatter, which annihilates matter upon contact, seems to be rare in the
There were only particles of matter and antimatter and light, uniformly filling
  all space.
Even closer to the beginning, complex physical processes set up a small excess
  of ordinary baryonic matter over antimatter.
Positrons are the antimatter equivalent of electrons, and are produced in a
  rare type of radioactive decay.
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