Arc
Curved segment in 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. 

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

Circumference
The perimeter of a circle.

Clockwise
An orientation, the direction in which the points are named when, if traveling along the line, the interior of the polygon is on the right.

Compass
A drawing tool used to draw circles at different radii.

Counterclockwise
In orientation, the direction in which points are named when, if travelling on the line, the interior of the figure is on the left side.

Minor arc
An arc whose endpoints form an angle less than 180 degrees with the center of the circle.

Major arc
An arc whose endpoints form an angle over 180 degrees with the center of the circle.

Pi
Greek letter used in geometry to represent the ratio of the circumference to the diameter and whose value is approx. 3.1416 (irrational number)

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.

Sector
Part of a circle containing its center and an arc.

Semicircle
An arc whose central angle is a right angle.