caltrop

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Any of various plants of the genera Tribulus and Kallstroemia, having spiny or tuberculate fruits.
2. n. A Mediterranean species of star thistle (Centaurea calcitrapa) naturalized in North America.
3. n. See water chestnut.
4. n. 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.

Century Dictionary and Cyclopedia

1. n. Formerly, a military instrument with four iron points disposed in such a manner that, three of them being on the ground, the fourth pointed upward. Caltrops were scattered on the ground where an enemy's cavalry were to pass, to impede their progress by wounding the horses' feet.
2. n. plural Broken pottery or coarse pots of easily broken earthenware, or other things adapted to wound horses' feet, used in place of caltrops proper.
3. n. In botany, a name of several plants. The name was applied first to the spiny heads or fruits of the plants, from their resemblance to the military instrument, and then to the plants themselves. The common caltrop or caltrops is Centaurea Calcitrapa (the star-thistle), found in waste places in the south of England. The heads are covered with long yellow spines. The name is also given to Tribulus terrestris, a plant of the Mediterranean region, with a spiny pentagonal fruit. The water-caltrop is Trapa natans, the fruit of which has several horns formed of the indurated lobes of the calyx.
4. To entangle with caltrops.
5. n. In the nomenclature of the spicular elements of sponges, a tetraxial spicule having the form of caltrop, with four equal simple smooth arms radiating from a central point.
6. n. plural In entomology, the short, sharp, curved spines which occur in scattered groups in the integument of certain lepidopterous larvæ of the family Limacodidæ, and which are responsible for the urticating effect produced on the human skin by these larvæ.

Wiktionary

1. n. weaponry A small, metal object with spikes arranged so that, when thrown onto the ground, one always faces up as a threat to passers-by.
2. n. colloquial The starthistle, Centaurea calcitrapa, a plant with sharp thorns.
3. n. A flowering plant, Tribulus terrestris, in the family Zygophyllaceae, native to warm temperate and tropical regions of the Old World.

GNU Webster's 1913

1. n. (Bot.) A genus of herbaceous plants (Tribulus) of the order Zygophylleæ, having a hard several-celled fruit, armed with stout spines, and resembling the military instrument of the same name. The species grow in warm countries, and are often very annoying to cattle.
2. n. (Mil.) An instrument with four iron points, so disposed that, any three of them being on the ground, the other projects upward. They are scattered on the ground where an enemy's cavalry are to pass, to impede their progress by endangering the horses' feet.

WordNet 3.0

1. n. a plant of the genus Trapa bearing spiny four-pronged edible nutlike fruits
2. n. Mediterranean annual or biennial herb having pinkish to purple flowers surrounded by spine-tipped scales; naturalized in America
3. n. tropical annual procumbent poisonous subshrub having fruit that splits into five spiny nutlets; serious pasture weed