/euh nuy'euh lay"sheuhn/ , n.
1. the act or an instance of annihilating.
2. the state of being annihilated; extinction; destruction.
3. Physics.
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.
[ 1630-40; ( annihilation- (s. of annihilatio ). See ANNIHILATE, -ION ]