“Apposition: Two consecutive, juxtaposed nouns or noun phrases are in apposition when they refer to the same person or thing, and when either can be omitted without seriously changing the meaning or the grammar of a sentence. Mrs. Thatcher and the British Prime Minister are in apposition in Mrs. Thatcher, the British Prime Minister, became leader of the Tory party in 1975. Here, both Mrs. Thatcher became leader…and The British Prime Minister became leader…could serve equally well alone. The term is often used when these criteria only partly apply, some grammarians using terms like partial or weak apposition to distinguish various types of lesser acceptability: ‘The heir to the throne arrived, Prince Charles’ (where only the second noun phrase can be omitted).
Excerpted from: McArthur, Tom. The Oxford Concise Companion to the English Language. New York: Oxford University Press, 2005.