A round straight line with a hole in it.

A continuous line or the plane bounded by such a line, in which every point of the line is equidistant from the central point lying on the plane.

A circle is commonly described by its radius, a straight line extending from the center of the circle to any point on the perimeter; and its diameter, a straight line extending from a point on the perimeter, through the center, to a point on the perimeter on the other side of the circle or it is expressed as "twice the radius".