A term for any time that the quarterback tries to complete a pass for a significant distance down the field. A bomb is usually an attempt to gain a large amount of yardage in a single play, and potentially score a touchdown. Bombs are used rarely because they do not work very often due to the difficulty of completing a pass accurately downfield against coverage.
The bomb is a major play for the offense if successful. Typically, a team will run a player on a deep route down the field in a manner that gets the receiver open against the defense. The two major problems that can limit the effectiveness of a bomb is that the defense will have a deep safety back to assist against the deep throw, and that placing a ball accurately at that distance is very difficult. Only certain quarterbacks have the arm strength and accuracy to throw the bomb.