infidelity

Etymology

From Middle French infidélité, from Latin infidelitas. Equivalent to infidel + -ity.

noun

1. Unfaithfulness in a marriage or an intimate (sexual or romantic) relationship: practice or instance of having a sexual or romantic affair with someone other than one's spouse, without the consent of the spouse.
   Your friends tell you rumors about your girlfriend's infidelity or you remember being broken up around the time the baby was conceived. 2013, William G. Staples, Everyday Surveillance: Vigilance and Visibility, page 155
2. Unfaithfulness in some other moral obligation.
   It was disastrous that England's infidelity towards Frederick the Great — which no one, not even a German, condemned more strongly than did William Pitt — had to affect one of the most popular heroes of our national history. 1937, Arnold Oskar Meyer, England in German opinion throughout the centuries, page 6
3. Lack of religious belief.
   The means used to this purpose are partly didactical, and partly protreptical; demonstrating the truth of the gospel, and then urging the professors of those truths to be stedfast in the faith, and to beware of infidelity. 1674, Seth Ward, Seven Sermons