Actually the definition of "down" was a bit scanty. It is also a verb, as in to down the ball, which means to bring an end to a play either by kneeling while in possession of the ball or, if you're the querterback, deliberately throwing an incomplete pass. (The latter also stops the clock.) To down the ball also may be used to describe when a kicking team, such as on a punt, grabs the ball before any of the receiving team's players do. For example, a punting team would rather down the ball on their opponents 1 yard line rather than let it go into the end zone thus resulting in a touchback.