What is an onside kick?

Which defensive position lines up deep in the secondary as the last line of defense? What is it called when a defender tackles the quarterback behind the line of scrimmage? What is it called when a defender catches a pass thrown by the offense? What is a "pick-six"? What is the name for a defensive play where extra rushers go after the quarterback? What is the term for a quarterback running the ball himself on a short-yardage push? What is a "flea flicker"? What is it called when the ball carrier loses possession of the ball?