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.

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.

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.

Secant to a circle
A line that intersects the circle in two points.

Semicircle
An arc whose central angle is a right angle.

Tangent to a circle
A line that intersects the circle in just one point, called point of tangency.