The curved line that traces the boundary or perimeter of a circle is called the circumference.
A straight line that touches the circle at just one point is called a tangent.
See the related link, 'The Parts of the Circle', below this answer.
A cardioid is the line a point on the edge of a circle traces when the circle is rolled around a circle of equal radius. A nephroid is the line a point on the edge of a circle traces when the circle is rolled around a circle of twice the radius.
That's known as the circumference.
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If you draw a line from the center of a circle to the edge of a circle, you have drawn the radius of the circle. If you draw a line from the edge of a circle through the center of the circle and on to the edge on the opposite side, you have drawn the diameter of a circle. The radius of a circle is one half the diameter of a circle.
A straight line from the center of a circle to the outer edge is called a radius.
The diameter of a circle is the length of a straight line from a point on the edge of the circle passing through the center of the circle and onto a point on the opposite edge of the circle. The radius of a circle is the length of a straight line from the center of a circle to a point on the edge of the edge of the circle. So the radius of a circle is half its diameter, in this case 8cm.
It is the radius of a circle
The line from the center of a circle to the edge is called a radius. The radius can be used in pi and a lot of other things. If you use the radius the wrong way, your answer will be wrong. Most people call it the " line from the center of a circle to the edge of a circle " but it's called the radius.
A tangent to a circle is a line which touches the circle once. That is, it does not pass through the circle, which would mean intersecting it twice. A way to form a tangent is draw any line from the centre point of a circle to its edge. A line on the edge perpendicular (at 90 degrees to) this line will be a tangent.