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.