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 = $\frac{c}{a}$ satisfies 0 < e < 1.