Unfaithfulness in a marriage or an intimate relationship: practice or instance of having a sexual or romantic affair with someone other than one's spouse, without the consent of the spouse.

Rejecting a specific religion. / Of, characteristic of, or relating to unbelievers or unbelief.

(now usually derogatory) One who does not believe in a certain religion. / (now usually derogatory) One who does not believe in a certain principle.

plural of infidelity

plural of infidel