The sweeper is a defensive position in football, so called because their job is to 'sweep up' the ball before it can go into the goal. It was most commonly used by football teams in Europe until the 1990s. While the position usually has solely defensive duties, Franz Beckenbauer, one of the most notable sweepers, became famous for adding an offensive component to the role.
It is generally considered a highly specialised position. The sweeper is usually placed between the goal keeper and the defensive line.
The sweeper is sometimes also called 'libero' from the Italian for 'free'.