The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

caltrop
    n 1: tropical annual procumbent poisonous subshrub having fruit
         that splits into five spiny nutlets; serious pasture weed
         [syn: caltrop, 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, caltrop]
    3: Mediterranean annual or biennial herb having pinkish to
       purple flowers surrounded by spine-tipped scales; naturalized
       in America [syn: star-thistle, caltrop, Centauria
       calcitrapa]