Gray water refers to wastewater generated in households or office buildings from flows without faecal contamination, i.e. all flows excluding sewage from toilets. Greywater sources include sinks, showers, bathtubs, washing machines or dishwashers. Because gray water contains fewer pathogens than domestic wastewater, it is generally safer and easier to treat and reuse on site for toilet flushing, landscape or crop irrigation, and other non-potable uses.

The reuse of gray water in urban water systems provides significant benefits both to the water supply subsystem by reducing the demand for drinking water, and to the wastewater subsystem by reducing the amount of wastewater needed for transport and treatment.