Radius

Any line segment from the center to a point on the circle, or its length. These two meanings are normally distinguished by context: A radius generally refers to a line segment, while the radius refers to the measurement.

Chord

A line segment whose end points lie on the circle. Chords may or may not pass through the center.

Diameter

A chord that passes through the center of a circle, or its length. As with radius, the two meanings are distinguished by context: A diameter is usually a chord through the center of the circle, while the diameter is the measurement. A diameter is a maximal length chord. The diameter of a circle is twice its radius.

Tangent

A line that passes through exactly one point on the circle.

Secant

A line that passes through two points on the circle.

Circumference

The length of the perimeter (boundary) of a circle. It is calculated as $C\= 2 \mathrm{\π} r$, where $r$ is the radius.

Arc

A part of the circumference from one point to another. Its length can be calculated by $a\=\mathrm{\θ}\cdot r$, where $\mathrm{\θ}$ is the angle in radians between the radii determined by the two end points, and $r$ is the radius of the circle. (Note that two points on the perimeter of a circle define two arcs, whose union is the entire perimeter. Which of these two arcs is intended must be made clear by the context.)

Sector

The region bounded by the two radii and the arc determined by two points on the perimeter of a circle.

Segment

The region bounded by the chord and the arc determined by two points on the perimeter of a circle.

Area

The area of a circle is calculated as $A\=\mathrm{\π} r^2$, where $r$ is the radius of the circle.