 A circle is defined by it’s center and all points equally distant from that center.  You name a circle according to it’s center point.  The radius of a circle is the distance from it’s center to any point on the circle.  A chord is a segment whose endpoints lie on the circle.

 The diameter of circle is twice the radius.  The diameter is the longest chord.  A central angle is an angle whose vertex is at the center of the circle.

 A tangent is a line that touches the circle at only one point.  A secant is a lines that passes through the circle touching it in exactly two points.  Line AB is a secant line.  Line CD is a  tangent line.

 The measure of a minor arc is equal to the measure of its central angle.  A minor arc measures less than 180°  When naming a minor arc, we use two letters and the arc symbol.  For example minor arc AB, we use the notation

 A major arc measures more than 180  It’s measure is 360 – the minor arc  When naming a it you must use 3 points on the arc to show it is major along with the arc symbol.  For example, major arc ADB is noted

 A semicircle is half the circle. Its endpoints are the endpoints of a diameter.  You must use three letters to name a semicircle.

 When talking about the degree measure of an arc, we use the notation

 The measure of an arc formed by two adjacent arcs is the sum of the measures of the two arcs

 Congruent circles have the same radius.  Congruent arcs have the same measure and lie in the same circle or congruent circles.