focus (of a conic)

A focus is one of one or two fixed points used to define a conic section by a distance rule: an ellipse's foci are the two points whose distances from any point on the curve always sum to a constant; a hyperbola's foci are the two points whose distances always differ by a constant; a parabola's single focus is the point every one of its points stays equally far from as it does from the directrix.