A football is a ball inflated with air that is used to play one of the various sports known as football. In these games, with some exceptions, goals or points are scored only when the ball enters one of two designated goal-scoring areas; football games involve the two teams each trying to move the ball in opposite directions along the field of play.
The first balls were made of natural materials, such as an inflated pig bladder, later put inside a leather cover, which has given rise to the American slang-term "pigskin". Modern balls are designed by teams of engineers to exacting specifications, with rubber or plastic bladders, and often with plastic covers. Various leagues and games use different balls, though they all have one of the following basic shapes:
- a sphere: used in association football and Gaelic football
- a prolate spheroid (elongated sphere)
The precise shape and construction of footballs is typically specified as part of the rules and regulations.
The oldest football still in existence, which is thought to have been made circa 1550, was discovered in the roof of Stirling Castle, Scotland, in 1981. The ball is made of leather (possibly from a deer) and a pig's bladder. It has a diameter of between 14–16 cm (5.5–6.3 in), weighs 125 g (4.4 oz) and is currently on display at the Smith Art Gallery and Museum in Stirling.