A seam pass in football is a pass that is thrown to a receiver running in the 'seam', or between multiple defenders. The seam pass is relatively easy to execute, requiring a short throw from the quarterback and little change of direction from the receiver.
The seam route is generally run by the slot receiver who is positioned between the offensive line and the wide receiver lined up near the numbers on the field. Once the ball is hiked, the slot receiver runs upfield in a straight, vertical line. Timing is everything when it comes to the seam route, as the quarterback needs to throw the ball once the slot receiver is approximately 10 yards past the line of scrimmage where the play began. This 10 yard range usually represents a "soft spot" in the opposing defense, falling directly in between the deep and intermediate zones of the field that are occupied by defenders.
The seam pass works only when the defense is in zone coverage. In zone coverage, defenders are assigned a portion of the field to guard, rather than an individual player on the offense. There are many different variations of zone coverage, however the seam route works best against a 'cover 3' defense, in which there are 3 defenders (free safety, two cornerbacks) responsible for deep coverage while the remaining defenders are tasked with the intermediate portion of the field.
When executed properly, the seam pass can leave the defense scrambling. Once the pass is completed, the defense is generally left with two options: