Definition of an illegal double pass
In the National Football League, a penalty is called any time two different players throw the ball forward during the course of a single play. This is punishable with a five-yard penalty.
Example of an illegal double pass
If a quarterback starts a play at the 20 yard line and throws a pass to a wide receiver at the 25, that receiver cannot turn around and throw the ball to another player closer to the opponent's end zone than he is--say, a tight end at the 40-yard line. This is a violation of the NFL Double Pass Rules, and it will draw a penalty flag from the officials.
Definition of a legal double pass
There are some instances in which the ball can be thrown twice in a single play. The caveat is that one of the two players throwing the ball must make sure that the pass travels laterally along the field, or it is caught by a player behind him.
Example of a legal double pass
When a play starts, a wide receiver can take several steps backwards and catch a pass from the quarterback. He is then allowed to throw the ball deep, since the original throw does not qualify as a forward pass.
Definition of a forward pass
A forward pass is when an offensive player (usually the quarterback) throws the ball forward. Only one forward pass is allowed during the course of a single play, starting with the snap and ending with the referee blowing his whistle to signify a stoppage of play. There are other factors that must be met for a forward pass to be legal, but none of them apply to the discussion of NFL Double Pass Rules.