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Radius

a segment whose endpoints are at the center of the circle and on the circle

Chord

a segment whose endpoints are points on the circle

Diameter

a chord that passes through the center of the circle (the longest chord in a circle)

Secant

a line that intersects the circle in exactly two points

Tangent

a line that intersects the circle at exactly one point

Concentric Circles

circles on the same plane that have a common center

Central Angle

an angle with its vertex at the center of the circle

Arc

an unbroken part of a circle, any two points on a circle are the endpoints of the arc between them

Minor Arc

an arc measuring less than 180º; minor arcs are named using the arc's endpoints with an arched hat; the measure of a minor arc is equal to the endpoints of its central angle

Major Arc

an arc measuring more than 180º, but less than 360º; major arcs are named for the endpoints and a point between them; the measure of a major arc is equal to 360º minus the measure of its minor arc

Semicircle

an arc whose endpoints are on a diameter of the circle; a semicircle also uses three letter notation; a semicircle measures 180º

Adjacent Arcs

two arcs in the same circle with exactly one point in common (an endpoint)

Intercepted Arc

the arc formed by intersecting an angle with a circle (the arc cut off by an angle)

Inscribed Angle

an angle whose vertex is on the circle and whose sides are chords of the circle