Wastewater is water generated after the use of freshwater, raw water, drinking water or saline water in a variety of deliberate applications or processes.^: 1 Another definition of wastewater is "Used water from any combination of domestic, industrial, commercial or agricultural activities, surface runoff / storm water, and any sewer inflow or sewer infiltration".^: 175 In everyday usage, wastewater is commonly a synonym for sewage (also called sewerage, domestic wastewater, or municipal wastewater), which is wastewater that is produced by a community of people.

As a generic term wastewater may also be used to describe water containing contaminants accumulated in other settings, such as:

• Industrial wastewater: waterborne waste generated from a variety of industrial processes, such as manufacturing operations, mineral extraction, power generation, or water and wastewater treatment.
• Cooling water, released with potential thermal pollution after use to condense steam or reduce machinery temperatures by conduction or evaporation.
• Leachate: precipitation containing pollutants dissolved while percolating through ores, raw materials, products, or solid waste.
• Return flow: the flow of water carrying suspended soil, pesticide residues, or dissolved minerals and nutrients from irrigated cropland.
• Surface runoff: the flow of water occurring on the ground surface when excess rainwater, stormwater, meltwater, or other sources, can no longer sufficiently rapidly infiltrate in the soil.
• Urban runoff, including water used for outdoor cleaning activity and landscape irrigation in densely populated areas created by urbanization.
• Agricultural wastewater: animal husbandry wastewater generated from confined animal operations.