An ellipse is the locus of a point P such that the sum of its distances from two fixed points (foci F1 and F2) is constant: PF1 + PF2 = 2a. The standard equation is x^2/a^2 + y^2/b^2 = 1 where a > b > 0. Here a is the semi-major axis, b is the semi-minor axis, and c = ae is the distance from centre to focus. The relation a^2 = b^2 + c^2 holds (contrast with hyperbola where c^2 = a^2 + b^2). The eccentricity e = c/a satisfies 0 < e < 1.