The set of points in a plane that are equidistant from a given point.

This is the distance around the outside of a circle.

This is the ratio of the circumference of a circle to its diameter.

This is a line segment between two points on the circle or sphere which passes through the center.

This is a line segment between the center and a point on the circle or sphere.

This is found by multiplying pi by the radius squared.

This of a circle is a segment that begins and ends on the circle; a segment whose endpoints are on the circle. It does not have to pass through the center.

This is a line, ray, or segment that intersects a circle at two points.

A line that intersects a circle in exactly one point.

An angle in a circle whose vertex is at the center of the circle. For trig functions, an angle in the unit circle whose vertex is at the center.

This is a conic section. The standard form is (x - h)squared + (y - k)squared = (r)squared.

A polygon whose vertices lie on a circle.

An angle whose vertex lies on the circle. The measure of this angle is one-half the measure of the intercepted arc.