antimatter

an·ti·mat·ter

noun \ˈan-tē-ˌma-tər, ˈan-ˌtī-\

Definition of ANTIMATTER

: matter composed of antiparticles

First Known Use of ANTIMATTER

1950

Rhymes with ANTIMATTER

for that matter, pitter-patter, printed matter, subject matter

antimatter

noun    (Concise Encyclopedia)

Substance composed of elementary particles having the mass and electric charge of ordinary matter (such as electrons and protons) but for which the charge and related magnetic properties are opposite in sign. The existence of antimatter was posited by the electron theory of P.A.M. Dirac. In 1932 the positron (antielectron) was detected in cosmic rays, followed by the antiproton and the antineutron detected through the use of particle accelerators. Positrons, antiprotons, and antineutrons, collectively called antiparticles, are the antiparticles of electrons, protons, and neutrons, respectively. When matter and antimatter are in close proximity, annihilation occurs within a fraction of a second, releasing large amounts of energy.