The Collaborative International Dictionary of English v.0.48:

Twinning \Twin"ning\, n. (Crystallog.)
   The assemblage of two or more crystals, or parts of crystals,
   in reversed position with reference to each other in
   accordance with some definite law; also, rarely, in
   artificial twinning (accomplished for example by pressure),
   the process by which this reversal is brought about.
   [1913 Webster]

   Polysynthetic twinning, repeated twinning of crystal
      lamellae, as that of the triclinic feldspars.

   Repeated twinning, twinning of more than two crystals, or
      parts of crystals.

   Twinning axis, Twinning plane. See the Note under Twin,
      n.
      [1913 Webster]