Betrayal means "an act of deliberate disloyalty," like when your friend told other people all your secrets. What a betrayal!
Betrayal's root is betray, which comes from the Middle English word bitrayen — meaning "mislead, deceive." Betrayal has to do with destroying someone's trust, possibly by lying. If you start dating your best friend's girlfriend behind his back, that's an act of betrayal. Betrayal can also mean "helping an enemy," such as a person who gives secret information to a country that is at war with his or her own.