In mathematics, an ellipse (from the Greek ἔλλειψις, literally absence) is a locus of points in a plane such that the sum of the istances to two fixed points is a constant. The two fixed points are called foci (singular- focus). An alternate definition would be that an ellipse is the path traced out by a point whose distance from a fixed point, called the focus, maintains a constant ratio less than one with its distance from a straight line not passing through the focus, called the directrix.