granulocyte A polymorphonuclear white blood cell with granule-containingcytoplasm. In humans the granulocytes are also classified as polymorphonuclear leucocytes and are subdivided according to the staining properties of the granules into eosinophils, basophils and neutrophils, some invertebrate blood cells are also referred to, not very helpfully, as granulocytes. BTO:0000539 BrendaTissueOBO leukocyte