Unfaithful (adjective)

1. Not loyal or true to a promise, duty, or belief; not keeping one's word or promise.
2. Not remaining loyal to a sexual partner; engaging in extramarital sexual activity.

Origin:

Un + faithful.

Examples:

1. He was unfaithful to his wife and had an affair.
2. The company was unfaithful to its contract and was sued.
3. She felt betrayed when she found out that her husband was unfaithful.
4. The employee was unfaithful to the company and provided confidential information to a competitor.
5. He was unfaithful to his party and voted for the opposing candidate.