A circle is a shape consisting of all points in a plane that are at a given distance from a given point, the centre.
Radius is the distance between any point of the circle and the centre.
Diameter is a straight line segment whose endpoints lie on the circle and that passes through the centre.
$$C=2 \times \pi \times r= \pi \times d$$
where,
π— pi or Archimedes's constant is a mathematical constant, that is a ratio of a circle's circumference to its diameter, approximately equal to 3.14159265359;
r— circle's radius;
d— circle's diameter.
$$S=\pi \times r^{2}=\frac{\pi \times d^{2}}{4}$$
The area of as circle is ~78,539816339% (i.e π/4) of the area of a surrounding square.