diachronism Syllabification: (di·a·chron·ism)Pronunciation: /dīˈakrəˌnizəm/Definition of diachronism noun Geology the occurrence of a feature or phenomenon in different geological periods. Derivatives diachronousPronunciation: /-nəs/ adjective diachronouslyPronunciation: /-nəslē/ adverb diachronism in other Oxford dictionaries Definition of diachronism in the British & World English dictionary