Prion:

This is the protein that makes up the infectious agent claimed by a large number of groups now to be the infectious particle that transmits the disease from one cell to another and from one animal to another. It is made from the normal protein PrPc (the c stands for chromosomal) that is produced in small quantities on many cells and especially the lymphoid and nervous tissue cells. 

PROteinaceous INfectious agent. The prion theory suggests that the infective agent of CJD (and the other TSEs) is only composed of a protein and does not contain nucleic acid which would be necessary if the agent was a conventional virius. 

a term proposed by neurologist Stanley Pruisner to identify "small proteinaceous infectious particles which are resistant to inactivation by most procedures that modify nucleic acids." The term, Pruisner added, "underscores the requirement of a protein for infection; current knowledge does not allow exclusion of a small nucleic acid within the interior of the particle."