A curve formed by a section of a cone, when the cutting
   plane makes a greater angle with the base than the side of the cone
   makes. It is a plane curve such that the difference of the distances
   from any point of it to two fixed points, called foci, is equal to a
   given distance. See Focus. If the cutting plane be produced so as to
   cut the opposite cone, another curve will be formed, which is also an
   hyperbola. Both curves are regarded as branches of the same hyperbola.
   See Illust. of Conic section, and Focus.