I.an instrument resting on three of its iron prongs, while a fourth projected upward, thrown on the ground to impede an enemy's cavalry, a caltrop.
II. Transf., from its resemblance in form.
A. A kind of thorn or thistle, land-caltrops: Tribulus terrestris, Linn.; Verg. G. 1, 153; Ov. M. 13, 803; Plin. 21, 15, 54, § 91: “spinae et tribuli,” Vulg. Gen. 3, 18.—
B. A kind of water-plant bearing a prickly nut of a triangular form, water - chestnut, water - caltrops: Trapa natans, Linn.; Plin. 21, 16, 58, § 98.