Source : Webster's Revised Unabridged Dictionary (1913)

Horsetail \Horse"tail`\, n.
   1. (Bot.) A leafless plant, with hollow and rushlike stems.
      It is of the genus {Equisetum}, and is allied to the
      ferns. See Illust. of {Equisetum}.

   2. A Turkish standard, denoting rank.

   Note: Commanders are distinguished by the number of
         horsetails carried before them. Thus, the sultan has
         seven, the grand vizier five, and the pashas three,
         two, or one.

   {Shrubby horsetail}. (Bot.) See {Joint-fir}.

Source : WordNet®

horsetail
     n : perennial rushlike flowerless herbs with jointed hollow
         stems and narrow toothlike leaves that spread by creeping
         rhizomes; tend to become weedy; common in northern
         hemisphere; some in Africa and South America