n. [ Twin + 1st -ling. ] A young or little twin, especially a twin lamb. [ 1913 Webster ]

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 ]