In basketball, the portion of the key denoted by an arc in the painted area positioned four feet from the basket.
The restricted area exists to afford some protection to offensive players when they are driving to the basket. Its specific purpose is to prevent a secondary defender from moving in front of the basket if the ball handler has driven past his original defender. If either of a defensive player's feet is present in or above the restricted area, no charging foul can be called on an opposing offensive player during a collision
There are a number of exceptions to this rule. For instance, any player can be in the restricted area if the offensive player received the basketball in the low post (or lower defensive box). Similarly, since the restricted area does not extend from the bottom of the backboard to the baseline, a player driving the baseline also cannot be ruled for a charge in this manner. Also, if the secondary defender instead of blocking the lane decides to attempt to block the shot, it does not matter whether or not he is in the restricted area. Although the rules are strictly defined, it is ultimately up to the officials to determine how to rule any fouls that occur in the restricted area.
The restricted area is also sometimes referred to as the "restricted area arc."