Webster's Revised Unabridged Dictionary · 1913

Infidel

Infidel , adjective

[Latin infidelis; pref. in- not + fidelis faithful, from fides faith: compare French infidèle. See Fidelity.]

Not holding the faith; -- applied by Christians to one who does not believe in the inspiration of the Scriptures, and the supernatural origin of Christianity; used by Muslims to refer to those who do not believe in Islam.
The infidel writer is a great enemy to society. — V. Knox

Infidel , noun

One who does not believe in the prevailing religious faith; a heathen; a freethinker; -- used especially by Christians and Muslims.

Infidel is used by English writers to translate the equivalent word used Muslims in speaking of Christians and other disbelievers in Muslimism.