\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.

{Polysynthetic twinning}, repeated twinning of crystal
   lamell[ae], 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.