If we are considering prions in general, I disagree with the answer supplied by Gerardo Furtado.
Here is a definition taken from an article by Susan Lindquist:
[Prions are]...self-perpetuating and heritable protein conformations that cause multiple phenotypes,
represent an unusual mechanism of information transfer that occurs via protein instead of nucleic acid.
Prions are infectious agents...
If a prion is self-perpetuating or infectious then the normal form of such a protein is not a prion. In yeast the term prion is used to refer to the infectious form of a protein in a formal genetic sense. For example the [PSI+] prion was discovered as a genetic element and then subsequently found to be associated with the translation termination factor Sup35, a protein with a known cellular role. All yeast strains have Sup35 protein but only certain strains contain and transmit the prion form of this protein. So [PSI+] is referred to as the prion form of the protein.
The problem with interpreting the language used to describe the TSE prions is that they were discovered because of their prion activity so the normal form is referred to as prion protein (as far as I am aware the function of the normal form isn't known). This doesn't mean that this form of the protein IS a prion, merely that this protein is associated with the original prion phenomenon so is called 'prion protein'.