Cartesion oval

Cartesian \Car*te"sian\, a. [From Renatus Cartesius, Latinized
   from of Ren['e] Descartes: cf. F. cart['e]sien.]
   Of or pertaining to the French philosopher Ren['e] Descartes,
   or his philosophy.
   [1913 Webster]

         The Cartesion argument for reality of matter. --Sir W.
                                                  Hamilton.
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   {Cartesian coordinates} (Geom), distance of a point from
      lines or planes; -- used in a system of representing
      geometric quantities, invented by Descartes.

   {Cartesian devil}, a small hollow glass figure, used in
      connection with a jar of water having an elastic top, to
      illustrate the effect of the compression or expansion of
      air in changing the specific gravity of bodies.

   {Cartesion oval} (Geom.), a curve such that, for any point of
      the curve mr + m'r' = c, where r and r' are the distances
      of the point from the two foci and m, m' and c are
      constant; -- used by Descartes.
      [1913 Webster]