In football, any combination of vocal instructions or hand gestures or motions used to communicate in-game decisions or plays by coaches, quarterbacks or other designated players.
The vast majority of signals in football are issued by the quarterback on the field of play prior to a snap; hence the quarterback's traditional title of "signal caller." Although coaches also employ signals to communicate strategy and play decisions, the advent of radio communication has eliminated much of the need for signals coming from the sidelines. Short signals and direction from coaches to players on the field still occur often, and usually take the form of quick, small gestures that communicate a particular type of play or intent on the part of the coach. Following a huddle or during a no-huddle play, the quarterback will often motion with his arms and head while calling a "count", prior to the snap. If the quarterback needs to change a play at the line of scrimmage due to a particular formation or read, he will often use pre-determined signals to communicate the change to his offense.
This term is also used to refer to official hand signals used by referees during a game to indicate penalties, timeouts or the outcomes of plays.