To deliver or expose someone to an enemy by treachery or disloyalty. He was accused of betraying his country by providing secrets to the enemy.
To be disloyal to someone by breaking a promise or trust. She felt hurt when her friend betrayed her by revealing her secret.
To reveal unintentionally or disclose something that was meant to be kept secret. His nervous smile betrayed his true feelings about the situation.
To fail or desert someone, especially in a time of need. The team felt betrayed when their leader abandoned them during the crisis.