Waste is unwanted or undesired material left over after the completion of a process. "Waste" is a human concept: in natural processes there is no waste, matter is used in continuous cycles.

Waste can exist in any phase of matter (solid, liquid, or gas). When released in the latter two states, they are usually referred to as pollution.

See also: air pollution, water pollution, soil contamination, municipal waste management, toxic waste, nuclear waste, electronic waste, recycling, composting, product stewardship.