What is "cassinian oval"

Oval \O"val\, n. A body or figure in the shape of an egg, or popularly, of an ellipse.

Cassinian oval (Geom.), the locus of a point the product of whose distances from two fixed points is constant; -- so called from Cassini, who first investigated the curve. Thus, in the diagram, if P moves so that P A.P B is constant, the point P describes a Cassinian oval. The locus may consist of a single closed line, as shown by the dotted line, or of two equal ovals about the points A and B.