Pronunciation: (u-nī"u-lā'shun), [key]
1. the act or an instance of annihilating.
2. the state of being annihilated; extinction; destruction.
a. Also called pair annihilation. the process in which a particle and antiparticle unite, annihilate each other, and produce one or more photons. Cf. positronium.
b. the conversion of rest mass into energy in the form of electromagnetic radiation.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.