1911 Encyclopædia Britannica/Caltrop

CALTROP (from the Mid. Eng. calketrappe, probably derived from the Lat. calx, a heel, and trappa, Late Lat. for a snare), an iron ball, used as an obstacle against cavalry, with four spikes so arranged, that however placed in or on the ground, one spike always points upwards. It is also the botanical name for several species of thistles.