1) Obsolete scientific theory 2) Particle 
<molecular biology, virology> A proposed infectious particle composed of protein and a short sequence of nucleic acids that does not code for any proteins. ... (13 Oct 1997) ... 
A small item that may be the infective agent of a TSE. It contains protein and nucleic acid.
[n] - (microbiology) a hypothetical infectious particle thought to be the cause of scrapie and other degenerative diseases of the central nervous system
noun (microbiology) a hypothetical infectious particle thought to be the cause of scrapie and other degenerative diseases of the central nervous system; consists of nucleic acid in a protective coat of host cell proteins