Billboard

Full definition of 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.
(computer graphics) A sprite that always faces the screen, no matter which direction it's looked at from.