A circle is a plane figure bounded by one curved line, and such that all straight lines drawn from a certain point within it to the bounding line, are equal. The bounding line is called its circumference and the point, it's centre. A circle is the same as 360°. You can divide a circle into smaller portions. A part of a circle is called an arc, and an arc is named according to its angle.
Concentric circles. Two or more circles that have the same centre, but different radii. Congruent circles. Two or more circles with the same radius, but various centres.
Because of their symmetry, circles were seen as representations of the "divine" and "natural balance" in ancient Greece. Later on, the shape would become a vital foundation for the wheel and other simple machines. A focus on circles is evident among structures built throughout history. The centre of a circle is the point equidistant from the points on edge. Similarly, the centre of a sphere is the point equidistant from the points on the surface, and the centre of a line segment is the midpoint of the two ends. As the ultimate curvilinear shape, the circle embodies all of the attributes that attract us: it is a safe, gentle, pleasant, graceful, dreamy, and even beautiful shape that evokes calmness, peacefulness, and relaxation.