"Sign" shall mean any rigid or semirigid material, with or without advertisement displayed thereon, situated upon or attached to real property outdoors, primarily or principally for the purpose of furnishing a background or base or support upon which an advertisement may be posted or displayed. Such sign may be in the form of writing, printing, a picture, painting, an emblem, drawing or similar device intended to invite or to draw the attention of the public to any goods, merchandise, property, business, services, entertainment or amusement.