Source : Webster's Revised Unabridged Dictionary (1913)

Billboard \Bill"board`\, n.
   1. (Naut.) A piece of thick plank, armed with iron plates,
      and fixed on the bow or fore channels of a vessel, for the
      bill or fluke of the anchor to rest on. --Totten.

   2. A flat surface, as of a panel or of a fence, on which
      bills are posted; a bulletin board.

Source : WordNet®

billboard
     n : large outdoor signboard [syn: {hoarding}]