O.E. twinn "consisting of two, twofold, double," probably ultimately from P.Gmc. *twinjaz (cf. O.N. tvinnr, O.Dan. tvinling, Du. tweeling, Ger. zwillung), from PIE *dwisno- (cf. L. bini "two each," Lith. dvynu "twins"), from *dwi- "double," from base *dwo- "two" (see two). The
verb meaning "to combine two things closely" is recorded from c.1394. The noun developed from O.E. getwinn "double."
One of two offspring born of a single gestation. Identical twins result from the division of a fertilized egg. Fraternal twins result from the fertilization of two separate eggs at the same time.
A crystal structure consisting of two intergrown crystals that are mirror images of each other. Mineral twins can form as result of defective crystal growth in response to stress from rock deformation or during magma cooling.