unfidelity

Noun

infidelity (countable and uncountable, plural infidelities)

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, →ISBN, page 155

   Synonyms:

   Synonyms: adultery

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

   Synonyms:

   betrayal

3. (archaic) 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^([sic]) in the faith, and to beware of infidelity.1674, Seth Ward, Seven Sermons

   Synonyms:

   irreligion, faithlessness