A circle is a shape consisting of all points in a plane that are a given distance from a given point, the centre.
- A circle is the curve traced out by a point that moves in a plane so that its distance from a given point is constant.
- The distance between any point of the circle and the centre is called the radius.
- Specifically, a circle is a simple closed curve that divides the plane into two regions: an interior and an exterior.
- In everyday use, the term “circle” may be used interchangeably to refer to either the boundary of the figure, or to the whole figure including its interior; in strict technical usage, the circle is only the boundary and the whole figure is called a disc.
- A circle may also be defined as a special kind of ellipse in which the two foci are coincident and the eccentricity is 0, or the two-dimensional shape enclosing the most area per unit perimeter squared, using calculus of variations.