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Old English twinn "consisting of two, twofold, double," probably ultimately from Proto-Germanic *twinjaz (cf. Old Norse tvinnr, Old Danish tvinling, Dutch tweeling, German zwillung), from PIE *dwisno- (cf. Latin bini "two each," Lithuanian dvynu "twins"), from *dwi- "double," from root *dwo- "two" (see two). The verb meaning "to combine two things closely" is recorded from late 14c. The noun developed from Old English getwinn "double."
One of two offspring born at the same birth. adj.
Being two or one of two offspring born at the same birth.
Consisting of two identical or similar parts; double.
either of two young who are simultaneously born from one mother. Twinning, common in many animals, is of two biological kinds: the one-egg (monozygotic), or identical, type and the two-egg (dizygotic), or fraternal, type. The latter type is more usual and can be thought of simply as a litter of two. In humans, psychological studies of sets of identical twins, since they are genetically identical, have provided much otherwise unobtainable information on the relative effects of genetic endowment and environment. See also multiple birth.