The Collaborative International Dictionary of English v.0.48:

Prion \Pri"on\ (pr[=e]"[o^]n), n.
   any of several types of protein particle lacking nucleic
   acid, believed to be the cause of certain slow-developing
   infectious diseases such as scapie in sheep, and
   Creutzfeldt-Jakob disease and Kuru in humans. [informal]
   [PJC]

WordNet (r) 3.0 (2006):

prion
    n 1: (microbiology) an infectious protein particle similar to a
         virus but lacking nucleic acid; thought to be the agent
         responsible for scrapie and other degenerative diseases of
         the nervous system