A circle is a simple geometric shape defined as the set of all points in a plane that are equidistant from a central point, known as the center. The distance from the center to any point on the circle is called the radius. Circles are commonly found in nature and human-made objects, such as wheels and coins. The circumference is the distance around the circle, while the area is the space contained within it. The mathematical constant π (pi) is crucial in circle calculations, as it relates the circumference to the diameter, which is twice the radius.