Basophil: A type of leukocyte (white blood cell) with coarse bluish-black granules of uniform size within the cytoplasm and typically a bilobate (two-lobed) nucleus. Basophils are so named because their cytoplasmic granules stain with basic dyes. Basophils normally constitute 0.5 to 3% of the peripheral blood leukocytes. Basophils contain (and can release) histamine and serotonin. Also called a basophilic leukocyte.