B'nai mitzvah:  A bar mitzvah is a boy; a bat mitzvah is a girl.  B'nai mitzvah is the plural, although when speaking of only girls, the plural is "b'not mitzvah."

"Bar mitzvah" means, literally, "son of the commandments."  A boy becomes bar mitzvah at age 13, at which time he is supposedly responsible for observing all of the mitzvot, or commandments.