MILL BY-PRODUCT is a term used to describe the waste material produced as a result of industrial processes, such as steel production. The crossword solution for this term is SLAG, which is a type of by-product created when metal is smelted or melted down. SLAG is composed of impurities and non-metallic materials that are removed from the metal during the smelting process. It is usually a glassy, granular material that can be used in construction and other industrial applications.