syncytiumPronunciation: /sɪnˈsɪtɪəm/Definition of syncytium noun (plural syncytia /-tɪə/) Biology a single cell or cytoplasmic mass containing several nuclei, formed by fusion of cells or by division of nuclei. Embryology a structure composed of syncytia, forming the outermost layer of the trophoblast. Derivatives syncytial adjective Origin: late 19th century: from syn- 'together' + -cyte 'cell' + -iumsyncytium in other Oxford dictionaries Definition of syncytium in the US English dictionary