coccolithophorePronunciation: /ˌkɒkə(ʊ)ˈlɪθəfɔː/Definition of coccolithophore noun Biology a single-celled marine flagellate that secretes a calcareous shell, forming an important constituent of the phytoplankton. Order Coccolithophorida, phylum Haptophyta Derivatives coccolithophorid noun & adjective coccolithophore in other Oxford dictionaries Definition of coccolithophore in the US English dictionary