A caltrop (jack rock, star nail) is a device made up of four (or more) sharp nails or spines arranged in such a manner that one of them always points upward from a stable base (e.g., a tetrahedron or tetrapod). In the military context, caltrops serve to slow down the advance of horses, war elephants, wheeled vehicles and human troops.

Caltrops come in a wide range of sizes, from small caltrops only a few centimeters across — used by ninjas to deter humans in soft-soled shoes — to massive steel (Czech hedgehogs) or concrete devices (dragon's teeth) designed to wedge into tank treads. Scattered around an encampment at night small, sharp caltrops can cause attackers stealthily treading on them to raise their own alarm.

Modern police and security forces often use spike strips against vehicles with inflated rubber tires, a device similar in principle to a patch of caltrops.

The device shares its name with the caltrop, Tribulus terrestris (Zygophyllaceae), whose spiked seed case can also injure feet and puncture tyres. Compare also the Star thistle, Centaurea calcitrapa, whose Latin name calcitrapa means "foot trap".

Other usage

In role-playing games, some players refer to 4-sided dice as "caltrops" because of their size and shape and the tendency of such dice to fall and roll underfoot.

Students sometimes play practical jokes by twisting staples together to form a mini-caltrop, then placing it on a hard seat where the victim will sit on it. Such devices are often referred to as "ass scorpions"

Environmental activists

In the 1970s, activists in the United States deployed caltrops against the tires of logging truck. Earth First! quickly condemned the practice, seeing it as a hazard both to humans and to other animals.

The Earth First! committment to nonviolent resistance includes the non-use of caltrops; traditionally other direct action organizations and individuals have rejected the use of such devices.