Central Angle - An angle formed by two radii in a circle. In other words, an angle whose vertex is the center of the circle. The measure of a minor arc ids defined to be the measure of the central angle.
Arc - Any part of the circumference of a circle.
Minor arc - An arc that covers less than half of a circle. The measure of a minor arc is always less than 180°.
Major arc - An arc that extends to more than half of a circle. The measure of a major arc is always more than 180°.
Chord - A segment that intersects a circle in two places.
Inscribed angle - An angle in a circle formed by two chords. In other words, an angle whose vertex is on the circle.
Tangent Line (to a circle) - A line that touches a circle in exactly one one.