1. To harm, or be disloyal to a country, or another person, by helping an enemy or giving information that is confidential: There are many examples of presumed heroes betraying their country by a casual comment to the wrong person or people.
2. To deliver someone or something to an enemy: The code in which a letter was written that was intended to betray the secrets of a government.
3. To act in a way that is contrary to a promise that has been made: Marla was very upset when she found out that her friend had betrayed her by posting private photographs on the internet.
4. To reveal something, or to show something, often unintentionally: Bill didn't say anything, but the expression on his face betrayed his eagerness to go out with Mary.
5. Etymology: bitrayen, "to mislead, to deceive, to betray"; from Middle English bi-, be-, "thoroughly" + Old French traien, from Latin tradere "to hand over"; from trans-, "across" + dare, "to give".