noun
infidelity
in-fih-DEL-uh-tee/ˌɪnfɪˈdeləti/
Failure to remain faithful, especially to a romantic partner.
The concealed romantic meeting reveals the married person's infidelity.

Etymology
The pieces express the quality of being unfaithful. Alongside religious unbelief, the word denoted disloyalty to a person, first a sovereign and later a lover or spouse.
In context
I've told you many times, I have a girlfriend. And I don't like infidelity. Neither of us would want our partner to be taken away, right?
More examples
- She considers secret romantic messages a form of infidelity.
- The couple seeks counseling to rebuild trust after infidelity.
- His repeated infidelities finally end their relationship.
Forms
infidelity · infidelities
fidelity noun — Loyalty to a person or commitment.
Similar words
- unfaithfulness
- A close equivalent covering romantic betrayal and broken commitments.
- adultery
- Specifically sexual intercourse involving a married person and someone outside the marriage.
- disloyalty
- Broader betrayal of allegiance, not necessarily involving romantic relationships.