Pronunciation: (u-nī"u-lā'shun),[key] —n. 1. the act or an instance of annihilating.
2. the state of being annihilated; extinction; destruction.
3. Physics. a. Also calledpair 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.