pri ‧ on /ˈpraɪɒn, ˈpriː- $ -ɑːn/ BrE AmE noun [countable]
[ Date: 1900-2000 ; Origin: proteinaceous 'containing protein' (19-21 centuries) + infectious particle ]
a very small piece of ↑ protein that is thought to cause some brain diseases such as ↑ BSE