The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   2. A flat surface, as of a panel or of a fence, on which
      bills are posted; a bulletin board. especially, A large
      board out of doors and visible to passers-by, on which the
      space is rented for advertising purposes; also, the
      advertising displayed on such a board.
      [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

billboard
    n 1: large outdoor signboard [syn: billboard, hoarding]