What best describes an ellipse?

An ellipse is a closed, symmetrical curve that resembles a flattened circle. It is defined as the set of points where the sum of the distances from two fixed points, called foci, is constant. The shape of an ellipse is determined by its major and minor axes, with the longer axis being the major axis and the shorter one being the minor axis. Ellipses are commonly found in various contexts, including planetary orbits and optical systems.