The early billboards could be found on barns and brick walls, in popular alleyways and warehouses across the country from about the 1890s until the television age. . . . As time marched on, old buildings were torn down or the old signs were painted over. Yet a few remain, their lead lettering often serving as the sole reminder of the product or service they sold. Some are visible only after a rain, prompting the nickname "ghost signs."
—Kaitlin Gurney, "Sign, sealed, delivered," The News and Observer (Raleigh, NC), October 1, 1999

According to the [Society for Commercial Archeology], a sign may fall into several categories. It may be a landmark (at least 20 years old and of special significance because of its design, size or configuration). Or it could be historic (theater marquees, neon letters and wood signs whose craftsmanship and construction materials speak of earlier eras). Or it may be a "ghost sign" — a faded, painted sign, at least 50 years old, on an exterior building wall heralding an obsolete product, an outdated trademark or a clue to the history of the building's occupancy. These signs often reappear after a rainstorm or following the demolition of a neighboring building.
—Beth Sherman, "Design Notes," Newsday, June 1, 1989