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An incomplete pass is a term in American and Canadian football which means that a legal forward pass hits the ground before a player on either team gains possession. For example, if the quarterback throws the ball to one of his wide receivers, and the receiver either does not touch it or tries to catch it unsuccessfully, it is ruled as an incomplete pass. An incomplete pass causes the down to advance by one and the offensive team gains no yards. The game clock is stopped.
Becoming a fumble or interception 
If the receiver catches the ball and has possession of it, then loses control of it for any reason and a player from the other team catches it a fumble is called. In the NFL, both feet must be in bounds and the player must have clear control of the ball for a number of seconds without taking pass interference into account. In other leagues, only one foot must be in bounds with clear possession carrying a lighter burden of proof.
If a member of the opposing team gains possession of the ball before it hits the ground, it is ruled an interception.
Pass interference 
If the receiver (or a defending player) is touched by a member of the opposing team in a way that prevents him from catching the ball, it is ruled pass interference, resulting in a penalty against the touching player's team. The exception to this rule is if the ball is deemed "uncatchable" by the referees, in which case a pass interference penalty is impossible.
Intentional incomplete pass 
Often, whenever defensive players blitz the quarterback, to avoid a sack, the quarterback will do what is known as "throwing the football away". This is a deliberate incomplete pass thrown off of the field in order to avoid loss of yardage. The main difference between this and intentional grounding is that the quarterback must leave the "pocket" of offensive lineman and throw past the line of scrimmage, so there is no penalty issued. Spiking the ball is also considered to be a legal incomplete pass.
- NFL Rulebook: Rule 4, Section 4(f)
- NFL Rulebook: Rule 8, Section 2, Article 1, Item 3