is a closed curve in a plane. All of its points are an equal distance from its
center. That distance is called the radius
of the circle. A diameter is a line segment that has both of its endpoints on
the circle and passes through the center.
is a line segment with both endpoints on the circle. A portion of a circle
is called an arc.
A semicircle is an arc that is one-half of a circle.
around a circle is called its circumference.
each term to learn more.