Circumference

The special name given to the perimeter of a circle

Centre

The middle point of a circle

Diameter

Any straight line joinging 2 points on the circumference that passes through the centre.

Radius (radii)

a straight line joining the centre to the circumference

Arc

a curved line joining 2 points on the circumference

Sector

an area between two radii and an arc

Chord

Any straight line joinging 2 points on the circumference that does not pass through the centre.

Segment

the area between a chord and an arc

Tangent

a straight line which touches the circle at one point only (the point of contact)