twinning

1 : the act of producing or giving birth to twins Rates of fraternal twinning vary from population to population, and the tendency to produce fraternal twins is genetically transmitted through the mother's line.— Jane E. Brody The frequency of twinning among women who have already borne twins, the 'repeat frequency,' is significantly higher than in the general population.— P. Parisi et al.

2 : the coupling, association, or comparison of two similar people, groups, or things It also encourages twinning between rich and poor parishes.— Thomas J. Reese The rhapsodizing literature and the inherent twinning of Disney's theme parks and its new town drew people with unrealistic dreams.— Douglas Frantz et al.

3 : the assemblage of two or more crystals or parts of crystals such that they form a twin (see twin entry 1 sense 3) One of them, a spiral defect, typically occurs when atoms crystallize from a high-temperature vapor. In the other defect, known as twinning, atomic lattices that are mirror images of each other join at a common boundary.— R. Cowen The two-time Nobel laureate Linus Pauling, for example, championed an arrangement of ordinary crystals called twinning. Twinned crystals grow from separate origins and penetrate each other at odd angles, such as 72 degrees. This might produce a diffraction pattern with spurious fivefold symmetry, even though the underlying structure was conventional.— Hans C. Von Baeyer