Circles (from the Greek kirkos, meaning ring, from the ancient root ker, meaning "to turn") are symbols of infinity – a line that never ends. Circles are also efficient: they cover the maximum possible area for a given perimeter or have the minimum possible perimeter for a given area. Circles are important, 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.
A circle is a shape consisting of all points in a plane that are a given distance from a given point, the centre; equivalently it is the curve traced out by a point that moves in a plane so that its distance from a given point is constant. A chord of a circle is a straight line segment whose endpoints both lie on the circle. A line that joins two points on the circumference of a circle is called a chord. A chord that passes through a circle's centre point is the circle's diameter. Every diameter is a chord, but not every chord is a diameter.
A circle is a special kind of ellipse in which the eccentricity is zero, and the two foci are coincident. A circle is also termed as the locus of the points drawn at an equidistant from the centre. The distance from the centre of the circle to the outer line is its radius. Diameter is the line which divides the circle into two equal parts and is also equal to twice the radius.