A substance that forms at the same time as the main or desired product during a chemical reaction. It is usually of secondary economic significance to the main product although many industrial by-products have significant economic value and commercial use in their own right. The product from the cracking of oil is petroleum. Many other by-products are also formed that have economic value such as paraffin, lubricating oils, and other distillates. In some cases, the revenues from by-products can exceed the main product, such as, the extraction of gold from porphyry (i.e. a coarse-grained type of igneous rock) deposits. Compare co-product.