2. Someone who is marked or motivated by an extreme, unreasoning enthusiasm, as for a cause.
3. A fanatic, a zealot, a militant, a devotee; all refer to people who show more than ordinary support for, adherence to, or interest in a cause, a point of view, or an activity.
4. Marked by excessive enthusiasm for and intense devotion to a cause or idea.
5. Etymology: From about 1525, "insane person", from Latin fanaticus, "mad, enthusiastic, inspired by a god," originally, "pertaining to a temple," from fanum "temple," related to festus "festive". The current sense of "extremely zealous", especially in religion, is first attested in 1647. The noun is from 1650, originally in a religious sense, of nonconformists.
Fanatic and zealot both suggest excessive or overweening devotion to a cause or belief.
Fanatic further implies unbalanced or obsessive behavior or a wild-eyed fanatic. A zealot, only slightly less unfavorable by implication than the term fanatic, implies a single-minded partisanship; such as, "a tireless zealot for tax reform".
Militant stresses vigorous, aggressive support for or opposition to a plan or ideal and suggests a combative stance.
Devotee is a milder term than any of the previous terms, suggesting enthusiasm but not to the exclusion of other interests or possible points of view; for example, a jazz devotee.
Another perspective about the term fanatic
According to the simplest etymology, "fanatic" derives from the Latin fanum, "temple"; but the meaning "zealous" or "zealot" seems to derive from the peculiar behavior of priests who served the Roman war goddess Bellona at a fanum built by the military dictator Sulla in the first century B.C.
Every year the priests staged a festival during which they tore off their robes and hacked at themselves with axes, splattering blood everywhere. This behavior could only be a sign of divine inspiration, and so fanaticus came to mean something like "crazed by the gods".
When the word "fanatic" first appeared in English in the sixteenth century, it meant "crazed person", and then more specifically "possessed with divine fury".
"Religious maniac" is still the principal meaning of the term, but in the shortened form "fan", it also simply means, "devotee" or "adherent".