Tackle (American football)

To physically interfere with the forward progress of a player in possession of the ball, such that his forward progress ceases and is not resumed, or such that he is caused to touch some part of his body to the ground other than his feet or hands, or such that he is forced to go out of bounds. In any such case, the ball becomes dead, the down is over, and play ceases until the beginning of the next play.

A special tackle known as a quarterback sack[?], when it is the quarterback who is tackled.