Twinning can refer to different things depending on the context. Here are some definitions based on the search results:

• Biology: The bearing of two children at one birth.
• Crystallography: The assemblage of two or more crystals or parts of crystals such that they form a twin.
• Animation and film: Special effects that multiply the presence of the same actor or actress on screen; in animation, twinning refers to when two parts on opposite sides of a characters body move the same way.
• Education: A type of transnational education in which a student does part of their degree course in a local private college and part of it in an overseas institution tied to the college.
• Partnerships: A partnership that links two entities with shared characteristics to achieve a common goal.
• Materials science: A prominent mechanism of plastic deformation in some metallic materials, produced by mechanical deformation or as the result of annealing following plastic deformation.

In general, twinning refers to the coupling, association, or comparison of two similar people, groups, or things.