(Science: chemistry) 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 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.