Source : Webster's Revised Unabridged Dictionary (1913)

Caltrop \Cal"trop\, Caltrap \Cal"trap\, n. [OE. calketrappe,
   calletrappe, caltor (in both senses), fr. AS. collr[ae]ppe,
   calcetreppe, sort of thistle; cf. F. chaussetrape star
   thistle, trap, It. calcatreppo, calcatreppolo, star thistle.
   Perh. from L. calx heel + the same word as E. trap. See 1st
   {Trap}.]
   1. (Bot.) A genus of herbaceous plants ({Tribulus}) of the
      order {Zygophylle[ae]}, 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. (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.

Source : WordNet®

caltrop
     n 1: tropical annual procumbent poisonous subshrub having fruit
          that splits into five spiny nutlets; serious pasture
          weed [syn: {devil's weed}, {Tribulus terestris}]
     2: a plant of the genus Trapa bearing spiny four-pronged edible
        nutlike fruits [syn: {water chestnut}, {water chestnut
        plant}]
     3: Mediterranean annual or biennial herb having pinkish to
        purple flowers surrounded by spine-tipped scales;
        naturalized in America [syn: {star-thistle}, {Centauria
        calcitrapa}]