Waste (or wastes) are unwanted or unusable materials. Waste is any substance which is discarded after primary use, or is worthless, defective and of no use. A by-product by contrast is a joint product of relatively minor economic value. A waste product may become a by-product, joint product or resource through an invention that raises a waste product’s value above zero. Examples include municipal solid waste (household trash/refuse), hazardous waste, wastewater (such as sewage, which contains bodily wastes (feces and urine) and surface runoff), radioactive waste, and others.

There are many waste types defined by modern systems of waste management, notably including: Municipal waste includes household waste, commercial waste, and demolition waste Hazardous waste includes industrial waste Biomedical waste includes clinical waste Special hazardous waste includes radioactive waste, explosive waste, and electronic waste (e-waste)