Definition and basic elements of a circumference

What is a circumference?

Formally, a circumference is defined as the locus of points from the plane equidistant to another point, called the center of the circumference.

We must never confuse the concept of a circle with the concept of circumference, circumference is actually a curve that encloses a circle (the circumference is a curve, the circle is an area.)

Here we see an image of a circumferece.


Actually, more simply, a circle is the set of points in the plane that are all at the same distance from one central point, which we will call the center.

Basic elements


In the image above you can see all the elements that we are going to introduce:

  • Center: center point that is equidistant to all points belonging to the circumference.
  • Radius: line connecting the center with any point belonging to the circumference.
  • String: line connecting any two points of the circumference.
  • Diameter: longer line connecting two points of the circumference. There are infinite diameters and all pass through the center of the circumference.
  • Secant line: line that cuts through any two points of the circumference.
  • Tangent: line touching the circle at just one point and perpendicular to a radius.