Diction is the choice and use of words for the expression of ideas. The word comes from Latin dictio, which means "saying," "word," and which appears in such familiar terms as dictionary, dictator, and dictate. Diction has been broadened in meaning to refer to one's whole style of speaking and writing: "This speaker was distinguished for his forceful, precise diction." Vocabulary refers to the Complete stock of words used or known by an individual or nation: "Joan's German vocabulary is limited, but she has a wide-ranging knowledge of Italian." Vocabulary is a more embracing term than diction: the latter refers to only words and expressions chosen and used by a speaker or writer, but the former includes not only the terms one uses but those that are in his recognition and reading store of words as well.
Dictionary of problem words and expressions. Harry Shaw. 1975.