In basketball, the perimeter is the area inside the three point line that's not inside the paint. The area is also referred to as mid-range, and the perimeter shots can be called mid-range shots. For example, a long two pointer, where the player steps on the three point line is a perimeter shot. The perimeter is an important section of the basketball court, especially on offense; players must move in the perimeter and look to attack the basket from the area.
Players who use the perimeter area to score, either by shooting form there, or attacking the basket starting in the area. Perimeter players are usually shooting guards or small forwards who can shoot open shots and drive to the basket well.
The term perimeter defense can be used referring to a whole team or an individual player. It is a defense that does not allow opponents to score from the perimeter area.