The word circle is derived from the Latin word circus, which means "ring."

A ** circle** is the set of all points in a plane
that are equidistant from a given point P called the

The ** radius**
of a circle is a segment that has the center as one endpoint and
a point on the circle as the other endpoint.

Click here for an Intermath description of a circle.

You name a circle by its center point. The circle above is called circle P.

By the definition of a circle, any two radii
of the same circle are congruent. If two circles have the same
radius, then they are ** congruent
circles**. If two or more coplanar
circles share the same center, then they are called

An ** arc
of a circle** consists of two points
on the circle and the continuous part of the circle between the
two points. The two points are called the endpoints of the arc.

A ** semicircle** is an arc of a circle whose endpoints are the endpoints
of a diameter. A

You name minor arcs with the letters of the two endpoints of the arc. You name semicircles and major arcs with the letter of three points, the first and last letters are the endpoints and the middle letter is any other point on the arc.

Click here for investigations.