“Apposition: A syntactic relation in which an element is juxtaposed to another element of the same kind. Especially between noun phrases that do not have distinct referents: e.g. Lucienne is in apposition to my wife in Do you know my wife Lucienne? Thence of other cases where elements are seen as parallel but do not have distinct roles in a larger construction: e.g. Smith is said to be apposed to Captain in Do you know Captain Smith?”
Excerpted from: Matthews, P.H., ed. The Oxford Concise Dictionary of Linguistics. New York: Oxford University Press, 2014.