A forward pass, legal or illegal, is incomplete and the ball becomes dead when the pass touches the ground or goes out of bounds. It is also incomplete when a player in is the air possesses the pass and alights so that his first contact with the ground or with anything other than a player or game official is on or outside a boundary. When an incompletion occurs the down counts unless the pass is after a change of possession. If the pass is legal, the passing team next snaps the ball at the previous spot, unless lost after fourth down.
Tag: dead
Rule 5, Section 3, Article 4
A ball touching the goal-line plane, when it becomes dead is in the end zone, even though it is moving away from the nearer end line and his its foremost point in the field of play.
Rule 4, Section 2, Article 1
A game official shall indicate the ball remains dead by sounding his whistle immediately when a foul occurs before a snap or free kick.
The Zebra Files 