To be unfaithful means to break a promise or commitment you made to someone who trusted you. The word can describe breaking marriage vows, but it also applies to any serious betrayal of trust.

When someone is unfaithful, they've chosen to act against what they promised, usually in secret. A friend might be unfaithful by sharing secrets you told them in confidence. A knight in medieval stories who betrayed his lord was called unfaithful to his oath of loyalty.

The word carries weight because faithfulness (being loyal and true to your word) is one of the foundations of trust. When you promise to be someone's friend, partner, or teammate, you're saying, “you can count on me.” Being unfaithful breaks that assurance.

The opposite of unfaithful is faithful: keeping your promises, staying loyal, and being the person others can depend on. A faithful dog stays by your side. A faithful friend keeps your secrets. Someone who is unfaithful has damaged or destroyed that trust, often causing deep hurt that's difficult to repair. Trust, once broken, takes far longer to rebuild than it did to break.