Ellipse

Pin a string at two points, pull it taut with a pencil and go round: the pencil draws an ellipse, the curve whose distances to the two foci add up to the same length. Apollonius of Perga named it, from the Greek for an omission or falling short. Light leaving one focus is reflected by the curve to the other, and each planet’s orbit is close to an ellipse with the Sun at one focus.

x²/a² + y²/b² = 1

On mathcurve.com as Ellipse