| Part | Description | Example | Properties |
| Center | A point in a plane from which all points on the circle are equidistant |  O is the center |
A circle can have only one center |
| Radius | A line segment with one end point at the center and other end point on the circle |
 OP is a radius |
The radius is always half the diameter R = d/2. A circle can have an infinite radii. (plural of radius) |
| Diameter | A chord that passes through the center of a circle. |  AB is a diameter. |
A diameter is twice the radius d = 2r. It is always passes through the center. It is the longest chord of a circle. |
| Circum- ference |
The length of a boundary of a circle. |  O is the center |
Every point on the circumference is equidistant from the center. |
| Chord | A line segment whose end points are on the circle. |  PQ is a chord. |
A chord can be of any length.It may or may not pass through the center. Diameter is the longest chord. |
| Arc | A part of a circle between any two points on the circle. |  PQR is an arc. |
It can be of any length. The bigger arc is called major arc and the smaller arc is called minor arc. |
| Segment | A chord divides a circle into two parts called segments. |  PQR is an arc. |
PQR is a minor segment . RSP is the major segment. |
| Sector | The region enclosed by two radii and an arc. |  sector AOB |
The smaller sector is called minor sector and the bigger sector is called major sector. |
| Semicircle | Half circle. |  |
Diameter of a circle divides it into two semicircles. |
