From WordNet (r) 3.0 (2006) [wn]:
billboard
n 1: large outdoor signboard [syn: billboard, hoarding]
From The Collaborative International Dictionary of English v.0.48 [gcide]:
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]