A circle is the set of all the points in a plane that are equidistant from a given point. There are a few particularly notable features of a circle.

• Center - The given point from which all points of the circle are equidistant. Circles are often named by their center point.
• Radius - A segment that connects the center and any point on the circle. Its length is usually represented algebraically by $r.$
• Diameter - A segment whose endpoints are on the circle and that passes through the center. Its length is usually represented algebraically by $d.$
• Circumference - The perimeter of a circle, usually represented algebraically by $C.$

The following circle can be referred to as $\odot O,$ or circle $O,$ since it is centered at $O.$ In any given circle, the lengths of any radius and any diameter are constant. They are called the radius and the diameter of the circle, respectively. To find the circumference and the area of a circle with radius $r,$ the following formulas are used.

Formulas for a Circle With Radius $r$
Radius	$r=\frac{d}{2}$
Diameter	$d=2r$
Circumference	$C=\pi d$ $C=2\pi r$
Area	$A=\pi r^2$