Parts of a Circle
A circumference is the distance around a circle. A radius is the distance from the centre to any point on the circumference. A diameter is a chord that passes through the centre.

Pi
π, the Greek letter pi, is a special number often used in calculations involving circles. The value of π is approximately 3.14 (to 2 d.p.). 
Perimeter of a Circle
The perimeter of a circle is called the circumference The length of the circumference (C) C = 2πr = πd 
Area of a Circle
The area of a circle is given by the formula
A = πr^{2}