Definition from Wiktionary, the free dictionary
billboard (plural billboards)
- A very large advertisement along the side of a highway.
- (dated) A flat surface, such as a panel or fence, on which bills are posted; a bulletin board.
- (nautical) 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.
- (Can we find and add a quotation of Totten to this entry?)
- (computer graphics) A sprite that always faces the screen, no matter which direction it's looked at from.
Derived terms 
a large advertising sign