A by-product is something created unintentionally while making or doing something else. When a lumber mill cuts trees into boards, it produces sawdust as a by-product. The mill's goal is lumber, but sawdust happens anyway. Some by-products prove useful: that sawdust might be compressed into particleboard or sold as animal bedding. Other by-products create problems: factories manufacturing steel produce slag and emissions as by-products that must be carefully managed.

The concept extends beyond manufacturing. When you practice piano scales to improve your technique, a better musical ear becomes a happy by-product. When a city builds a new highway, increased traffic in nearby neighborhoods might be an unwanted by-product. Scientists studying one problem often make discoveries as by-products of their research: penicillin was famously discovered as a by-product of a different experiment.

The word suggests something secondary or incidental. Unlike the main product or goal, a by-product isn't the reason you started. But by-products can sometimes become just as important, or even more valuable, than whatever you set out to create in the first place.