- one of the evil spirits of traditional Jewish and Christian belief

- a person motivated by irrational enthusiasm (as for a cause)
"A fanatic is one who can''t change his mind and won''t change the subject"--Winston Churchill

- a cruel wicked and inhuman person

1. An implacable or malicious foe; one who is diabolically wicked or cruel; an infernal being; -- applied specifically to the devil or a demon.

"Into this wild abyss the wary fiend Stood on the brink of Hell and looked a while." -- Milton.

"O woman! woman! when to ill thy mind Is bent, all hell contains no fouler fiend." -- Pope.