When you have finished this page, try the Circles Quiz.

A circle is a plane figure with all points on the outside or circumference equally distant from the center of the figure.

Circle image with a diameter


A line that is drawn from a point on one side of the circle to a point on the other side and passes through the center is the diameter of the circle. The diameter is a line segment, and thus is named by the letters that are at each endpoint.

Circle with a diameter


A line segment drawn from the center of the circle to a outside point of the circle is called a radius (plural radii). The radius is named by the letters at its endpoints.

Circle with radius


A chord is a line segment that touches two points on the outside of the circle but doesn’t pass through the center.

circle with a chord


The measure of the distance around the outside of a circle or the circle’s perimeter is its circumference.

Circle circumference


A tangent is a line or line segment that touches the outside of the circle at a point, but does not pass through it in any way.

Circle with tangent line


A secant is a line that goes through or intersects a circle at two points and extends beyond the limits of the circle.

Circle with secant through it


An arc is a part of a circumference of the circle that has two endpoints on the circumference of the circle.

an arc