A caltrop is a device made up of four (or more) sharp nails or spines that are arranged in such a manner that one of them is always pointing upward (eg, a tetrahedron). In the military context, it has been used to slow down the advance of horses and wheeled vehicles. Caltrops come in a wide range of sizes, from small caltrops only a few centimeters across that were used by ninjas to deter humans in soft-soled shoes to massive steel or concrete devices designed to wedge into tank treads.
Modern police and security forces often use spike strips against vehicles with inflated rubber tires, a device similar to a patch of caltrops in principle.
The device is named for its resemblance to the spiked seed case of the caltrop, Tribulus terrestris (Zygophyllaceae), which can also injure feet and puncture tyres.