The word circle is of Greek origin and reference to circles is found as early as 1700 BC (BCE). In Nature circles would have been observed, such as the Moon, Sun, and ripples in water. The circle is the basis for the wheel, which, with related inventions such as gears, makes much of modern machinery possible. In mathematics, the study of the circle has helped to inspire the development of geometry, astronomy and calculus. In Bohr-Sommerfeld theory of the atom, electron orbit is modelled as circle.
A circle is the locus of a point in a plane which moves such that its distance from a fixed point in the plane is always a constant.
The fixed point is called the centre and the constant distance is called radius of the circle.