sesquicentenary Syllabification: (ses·qui·cen·ten·ar·y)Pronunciation: /ˌseskwisenˈtenərē/Definition of sesquicentenary noun chiefly British (plural sesquicentenaries) a sesquicentennial. adjective of or relating to a sesquicentennial. sesquicentenary in other Oxford dictionaries Definition of sesquicentenary in the British & World English dictionary