unwanted secondary product of a production process, manufacturing process or chemical reaction
A by-product is something that is made or produced while a product is manufactured in a factory. Usually, the by-product is not used, and it becomes waste that is put in a landfill or garbage dump. Sometimes by-products are toxic materials which are dangerous to humans, animals, and plants.
Some companies are trying to find ways to reuse or recycle the by-products, instead of putting them into landfill or garbage dumps as waste.
Animal by-products are produced by slaughterhouses (places designed to kill animals for food). Animal by-products are the parts of the animals that cannot be eaten by humans. Animal by-products are sometimes included in non-human foods, such as pet food or animal feed (for farm animals).