Any closed curve (similar to a figure eight) described by a Cartesian equation of the form (x^2 + y^2)^2 = a^2(x^2 - y^2)
A curve in the form of the figure 8, with both parts symmetrical, generated by the point in which a tangent to an equilateral hyperbola meets the perpendicular on it drawn from the center
a closed curve, similar to a figure eight, that is the locus of a point, the product of whose distances from two fixed points (called the foci) a distance 2a away is the constant a^2