The Collaborative International Dictionary of English v.0.48:

Porism \Po"rism\, n. [Gr. ? a thing procured, a deduction from a
   demonstration, fr. ? to bring, provide: cf. F. porisme.]
   1. (Geom.) A proposition affirming the possibility of finding
      such conditions as will render a certain determinate
      problem indeterminate or capable of innumerable solutions.
      --Playfair.
      [1913 Webster]

   2. (Gr. Geom.) A corollary. --Brande & C.
      [1913 Webster]

   Note: Three books of porisms of Euclid have been lost, but
         several attempts to determine the nature of these
         propositions and to restore them have been made by
         modern geometers.
         [1913 Webster] Porismatic