An Infidelity

1. 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.
2. Unfaithfulness in some other moral obligation.
3. Lack of religious belief.

(marital): adultery

(moral): betrayal

(religious): faithlessness

(moral): faithfulness

(moral): loyalty

(moral): fidelity