(an act of) having sex with someone who is not your husband, wife, or regular sexual partner. (Cambridge Dictionary)
The act or fact of having a romantic or sexual relationship with someone other than one’s husband, wife, or partner. (Merriam – Webster)
Unfaithfulness to a moral obligation: Disloyalty. (Merriam – Webster)
Lack of belief in a religion. (Merriam – Webster)
Infidelity occurs when a person who is married or in a steady relationship has sex with another person. (Collins Dictionary)
The action or state of being unfaithful to a spouse or other sexual partner. (Oxford Dictionaries)
Disbelief in a particular religion, especially Christianity. (Oxford Dictionaries)
Other words for Infidelity. Thesaurus.
How to Define Emotional Infidelity: Types of Cheating.