caltrop n tropical annual procumbent poisonous subshrub having fruit that splits into five spiny nutlets; serious pasture weed n a plant of the genus Trapa bearing spiny four-pronged edible nutlike fruits n Mediterranean annual or biennial herb having pinkish to purple flowers surrounded by spine-tipped scales; naturalized in America Synonym(s) devil's weed Tribulus terestris water chestnut water chestnut plant star-thistle Centauria calcitrapa