**difference**of the distances from any point on a hyperbola to each focus is constant and equal to the transverse axis $2a$.

ellipse: the

**sum**of the distances from any point on an ellipse to each focus is constant and equal to the major axis $2a$.

$c= \sqrt{a^2 - b^2}$ $c$: distance from the center to a focus

$e= \frac{c}{a}$ $e$: eccentricity; the larger the value of $e$, the straighter the hyperbola