Pronunciation: [ zel-uht ]
Definition: noun
a person who is fanatical and uncompromising in pursuit of their religious, political, or other ideals

Origin: Late Middle English; Greek
Etymology: via ecclesiastical Latin from Greek zēlōtēs, from zēloun ‘be zealous’, from zēlos (see zeal)
Usage:

Synonyms:
activist
diehard
extremist
fanatic
maniac
radical
Antonyms:
conservative
moderate
Example:
His immense enthusiasm toward his ideals easily qualified him as a zealot.