infidelity

noun3

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
2. •
   Unfaithfulness in some other moral obligation.
3. •
   (no longer used) Lack of religious belief.

   • The Churchman remarked that such incidents arose from the growing infidelity, while the Freethinker saw in them a reversion to superstition. — 1926