an incidental product deriving from a manufacturing process or chemical reaction, and not the primary product or service being produced. A by-product can be useful and marketable, or it can have negative ecological consequences.
see also: co-products, industrial symbiosis
note: A by-product's output is incidental, and its economic value can depend strongly on the regulatory environment. Normally, co-products and by-products are exclusive categories, but nowadays, this distinction is fading. For example, sulfuric acid and iron silicate are by-products of copper processing, but they can support/comply with sustainable production, the circular economy, and zero-waste processing. Those aspects are as important as their economic value.---