Radius: The distance from the centre to the circumference of a circle. It is half of the circle's diameter.

Diameter: The distance from one point on a circle through the centre to another point on a circle. It is also the longest distance across the circle.. It is twice the radius.

Circumference: The distance around the edge of a circle. It is a type of perimeter.

Chord: A line segment connecting two points on a curve.

Tangent: A line that just touches a circle at a point.

Arc: part of the circumference of a circle.

Sector: A 'pie-slice' part of a circle.

Segment: Part of a circle made when it is cut by a line.        

Sphere: A 3-dimentsional object shaped like a ball.

Cone: A solid object that has a circular base joined to a point by a curved side.