infidelity

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.

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.

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.