Pi, also written with the character π, is an irrational number that is approximately 3.14. When you multiply its diameter by pi, your get the distance around a circle. (The diameter is the distance of a line that goes from one point on the circle to the opposite point; the line will also go through the middle of the circle's area.) Pi times the radius squared will give you the area inside the circle. (The radius is the distance from the middle of the circle to any point on the circle or half the diameter.)