In football, a pair of six foot wide, painted parallel lines that mark the outermost boundaries of the field of play.
The sidelines enclose and separate the field of play from out of bounds territory. There are a number of rules involving the sidelines and constituting a play that is out of bounds. If a player catches or possesses of the football and is beyond or touches either sideline, he is considered to be out of bounds and the play is called dead. Likewise, once the ball touches or crosses the border of the field and the sideline it is also considered out of bounds. If an offensive player is determined to have gone out of bounds and returns to the field of play, it is illegal for that player to possess the ball, although a defensive player may do so and still legally make a tackle.
In addition to their literal definition, the "sidelines" also often refer to the immediate areas beyond the sidelines populated by team members and coaches during the game.