It is defined as the set of all points in a plane where the sum of the distance from two points, the foci, is the same. A special case of the ellipse is the circle, where the two points are coincident.
To construct an ellipse, place tacks at two points on a piece of paper with a string between them. Trace out the curve of the ellipse with a pencil which is always pushing against the string. The longer the string, the less eccentric the ellipse will be (and more circular).
The general algebraic formula for an ellipse is given by:
An ellipse is a conic section, the intersection of a plane and a cone, where the angle of the plane to the cone's axis is greater than the angle of the cone with its axis.
The terms eccentric and circular are antonyms.
A pair of line segments drawn from one focal point of an ellipse to the curve and from there to its other focal point form an angle whose bifurcating line is always perpendicular to the tangent of the curve. This feature of an ellipse has been exploited in rooms having elliptical walls or (more dramatically) elliptical ceilings. A whisper at one focal point is easily heard at the the other focal point, because the sound waves bounce off the walls and combine again at the other focal point.
Ellipses became important in astronomy in the early 1600s, when Kepler proved that planets revolving the sun always follow elliptical orbits, with the sun at one of the foci. This helped overturn the Ptolemaic theory, and led to Issac Newton's law of gravitation.