Merriam-Webster Collegiate Dictionary - caltrop

 also calthrop  noun  Etymology: Middle English caltrappe, alteration of calketrappe star thistle, from Old English calcatrippe, from Medieval Latin calcatrippa  Date: 15th century  1.  a. plural but singular or plural in construction star thistle 1  b. puncture vine; also any of various related herbs (genera Tribulus and Kallstroemia) of the same family (Zygophyllaceae, the ~ family)  2. a device with four metal points so arranged that when any three are on the ground the fourth projects upward as a hazard to the hooves of horses or to pneumatic tires