Radius: It is a straight line from the centre of the circle to any point on its circumference.
Diameter: It is a straight line, that passes through the centre of a circle, between any two points on its circumference.
Chord: It is a straight line between any two points on its circumference.
Tangent: It is a straight line that touches the circle at exactly one point on its circumference.
Semicircle: It is half of a circle or half of its circumference.
Arc: It is a part of the circumference of a circle.
- Major Arc (more than half of its circumference)
- Minor Arc (less than half of its circumference)
Segment: It is a part of a circle that is enclosed by an arc and a chord of the circle.
- Major Segment (more than half of the circle)
- Minor Segment (less than half of the circle)
Sector: It is a part of a circle that is enclosed by an arc and two radii of the circle.
- Major Sector (more than half of the circle)
- Minor Sector (less than half of the circle)
 |  |
Below shows a circle, with centre O.
The points A, B, C and X are points on the circumference of the circle.
You can:
- shift the point O to change the position of the circle
- shift the point X to change the size of the circle
To learn more about the various parts of a circle:
- check and uncheck the boxes
- shift the points A and B to change their positions
Note: To see the animation, click on the "play" button (at bottom left).
Download the GeoGebra file: http://www.geogebratube.org/material/download/format/file/id/41189
