American football, known as football in the United States, is a team sport played between two teams of 11 players with an oval ball on a rectangular field 120 yards long and 53.3 yards wide with goalposts at either end. The team in possession of the ball (the offense) attempts to advance down the field by running with or passing the ball. In order 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, but if they fail, they lose control of the ball to the opposing team.
The team that has scored the most points by the end of the game wins. The offense can score points by advancing the ball into the end zone (a touchdown) or by place or drop 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).
American football evolved from early forms of rugby (particularly rugby union) and association football (soccer), with the first game played on November 6, 1869. Rule changes from 1880 on by Walter Camp included the snap, 11-a-side teams and downs; further rule changes legalized the forward pass and created the neutral zone along the width of the football.
Today, American football is the most popular sport in the United States, where the National Football League (NFL) is the most popular league. The league's championship, the Super Bowl, is among the most-watched club sporting events in the world. College football is also popular in the United States, with the NCAA Division I Football Bowl Subdivision as the top level competition.