§ DICTIONARY · CONCEPT
focus
One of two interior points that define an ellipse; the central body sits at one focus in a Keplerian orbit.
§ 01
Definition
An ellipse has two foci, sitting on its long axis on either side of the center. They are the two points used in the defining property of the ellipse: the sum of distances from any point on the curve to the two foci is constant.
In a Keplerian orbit, one of the foci is occupied by the central body — the sun, or whatever massive object the orbit bends around. The other focus is empty. A circular orbit is the degenerate case where both foci coincide at the center.