Infidelity

noun voluntary sexual intercourse between a married person and someone other than their spouse

• Committing adultery is considered a serious breach of marital vows.
• I heard that John was caught in adultery with his coworker.
• Cheating on your spouse is just plain wrong, no matter how you spin it.
• In literature, adultery is often used as a symbol of betrayal and moral decay.

noun a person who acts dishonestly or unfairly in order to gain an advantage

verb to act dishonestly or unfairly in order to gain an advantage

• It is unethical to cheat on a test by looking at someone else's answers.
• I heard he tried to cheat on his girlfriend with her best friend.
• She's always trying to cheat her way out of doing any work.
• Trying to cheat the system will only lead to trouble in the long run.

adjective not loyal or faithful; betraying someone's trust or confidence

• The disloyal employee was eventually terminated for his lack of commitment to the company.
• I can't believe she was so disloyal to her best friend by spreading rumors about her.
• He's always been known as a backstabber, constantly being disloyal to his friends.
• The disloyal act of the sun hiding behind the clouds made the picnic attendees disappointed.

adjective showing a lack of trust or loyalty; not faithful

• The faithless politician was quickly ousted from office.
• I can't believe she turned out to be so faithless.
• He's such a faithless jerk, always breaking promises.
• The crumbling old building stood as a monument to the faithless passage of time.