The area of a basketball court outside the boundary lines. It is also a call made by referees when the ball crosses a boundary line. Every level of competition has rules which govern the dimensions of the court. These dimensions include two end lines and two sidelines, as well as the height of the rim, backboard, and other parts of the court. When a ball crosses a boundary determined to be outside the field of play, an out of bounds call is made. At some levels of play, a ball hitting the top of the backboard, or any hardware holding it up, is considered to be outside the field of play.
The idea of bounds on a court or field is not unique to basketball. In fact, most every sport includes some type of bounds which indicate where a player may actively participate in the competition. These bounds are important in basketball because the court is small but the game is typically fast paced. By establishing boundaries on either side of the court, the game stays organized and moving, and-as TV networks will tell you-easier to put on television.