American football, known in the United States as football, is a team sport. It is played by two teams, eleven players to a side, who advance an oval ball over a rectangular field that is 120 yards long by 53.3 yards wide and has goalposts at both ends. The team in possession of the ball (the offense) attempts to advance down the field by running with or passing the ball. To continue their drive, the offense must advance the ball at least 10 yards down the field in a series of four downs. If they succeed, they receive a new set of four downs to continue their drive. Otherwise, they lose control of the ball to the opposing team. The offense can score points by advancing the ball into the end zone (a touchdown) or by kicking the ball through the opponent's goalpost (a field goal), while the defense can score points by forcing an offensive turnover and advancing the ball into the offense's end zone or by tackling the ballcarrier in the offense's end zone (a safety). The team that has scored the most points by the end of the game wins.

American football evolved from early forms of rugby, particularly rugby union, and association football (soccer). The first game was played on November 6, 1869 under a set of rules resembling those of rugby and soccer. A set of rule changes instituted from 1880 onward by Walter Camp established the snap, eleven-player teams and downs. Later rule changes legalized the forward pass, created the neutral zone and specified the width of the football.