Circle

The word circle is of Greek origin, and references to circles are found as far back as 1700 BCE. Circles are common in nature too — the Moon, the Sun, ripples in water — and the circle underlies the wheel, which, together with related inventions such as gears, makes much of modern machinery possible. In mathematics, studying the circle has helped inspire the development of geometry, astronomy and calculus; in the Bohr–Sommerfeld model of the atom, an electron's orbit is modelled as a circle.

Definition 5.1. A circle is the locus of a point in a plane that moves so that its distance from a fixed point in the plane is always constant. The fixed point is the centre; the constant distance is the radius. …