Arc
The curved segment that is between two points in the circumference of a circle.

Arc length
The distance between an arc's endpoints along the path of the 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

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.