What does "twinning" mean?

Twinning: 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 lamellæ, 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.