Countable; refers to a person exhibiting extreme, uncritical zeal.
Definitions
A person who is extremely enthusiastic or devoted to a particular cause, especially in an uncritical or obsessive way. The stadium was packed with football fanatics waving banners and chanting.
A person whose zeal, especially for a religious or political cause, is extreme and irrational, sometimes leading to aggressive or violent behavior. The assassination was carried out by a fanatic who believed he was serving a higher purpose.