A circle can be drawn using a pair of compasses.
The distance set between the point and the pencil tip is called the radius.
The point marks the centre of the circle.
The distance straight across from one edge to the other, going through the centre, is called the diameter.
The diameter is twice as long as the radius.
The radius is half the length of the diameter.
When you draw a circle, the pencil line around the perimeter is called the circumference.
It is a curved line.