Arc
Curved segment in a circle.

Center of a circle
The point that all points in the circle are equidistant from

Central angle of a circle
An angle whose vertex is the center of the circle

Chord of a circle
A segment whose endpoints are on a circle

Circle
The set of points on a plane at a certain distance (radius) from a certain point (center); a polygon with infinite sides

Postulate
A statement assumed to be true without proof.

Proof
A sequence of justified conclusions used to prove the validity of an if-then statement.

Radii
Plural form of radius

Radius
The segment whose endpoints are any point on a circle or sphere and its center; the length of that segment

Semicircle
An arc whose central angle is a right angle