A metal device with four projecting spikes so arranged that when three of the spikes are on the ground, the fourth points upward, used as a hazard to pneumatic tires or to the hooves of horses.

http://www.thefreedictionary.com/caltrop

A caltrop (also known as caltrap, galtrop, cheval trap, galthrap, galtrap,[1] calthrop, crow's foot[2][3]) is an antipersonnel weapon made up of two or more sharp nails or spines arranged in such a manner that one of them always points upward from a stable base (for example, a tetrahedron). Caltrops were part of defenses that served to slow the advance of horses, war elephants, and human troops.

http://en.wikipedia.org/wiki/Caltrop

Rómában tribulus néven ismerték.
http://en.wikipedia.org/wiki/Roman_military_personal_equipme...