The Collaborative International Dictionary of English v.0.48:
Signboard \Sign"board`\, n. A board, placed on or before a shop, office, etc., on which ssome notice is given, as the name of a firm, of a business, or the like. [1913 Webster]WordNet (r) 3.0 (2006):
signboard n 1: structure displaying a board on which advertisements can be posted; "the highway was lined with signboards" [syn: signboard, sign]