n. Formerly, a military instrument with four iron points disposed in such a manner that, three of them being on the ground, the fourth pointed upward. Caltrops were scattered on the ground where an enemy's cavalry were to pass, to impede their progress by wounding the horses' feet.n. plural Broken pottery or coarse pots of easily broken earthenware, or other things adapted to wound horses' feet, used in place of caltrops proper.n. In botany, a name of several plants.To entangle with caltrops.n. In the nomenclature of the spicular elements of sponges, a tetraxial spicule having the form of a caltrop, with four equal simple smooth arms radiating from a central point.n. plural In entomology, the short, sharp, curved spines which occur in scattered groups in the integument of certain lepidopterous larvæ of the family Limacodidæ, and which are responsible for the urticating effect produced on the human skin by these larvæ.