A physical gesture by a team member or coach on either the offensive or defensive side of a football team used to communicate or supplement a play call or strategy on the field.
Hand signals are most often used in conjunction with audibles called out by the quarterback on offense. When a quarterback needs to change a play prior to the snap, he uses predetermined hand signals while calling out audibles. Signals are sometimes complex, based on the number of fingers displayed or the position of the hands, or they can be as simple as pointing towards opposing players and team members to indicate threats or potential routes.
Hand signals also sometimes serve as a helpful substitute for audibles, especially for quarterbacks visiting a hostile stadium where crowd noise can be a significant factor. In addition to quarterbacks, coaches of both the defense and offense will often communicate using hand signals in addition to any vocal instructions. Defensive players also use hand signals as they respond to changes by the offense prior to the snap, most often to point out changes in the formation or to instruct another defender to change coverage.