Greywater is defined by the Uniform Plumbing Code (UPC) as untreated household waste water which has not come into contact with toilet waste. Greywater includes used water from bathtubs, showers, bathroom wash basins, and water from clothes-washer and laundry tubs. It must not include waste water from kitchen sinks or dishwashers.
Greywater systems reuse this water for functions such as flushing toilets or irrigation, resulting in a significant reduction in potable (drinkable) water usage that would otherwise be needed for these functions.
In LEED, greywater systems may be used to earn a credit in the Water Efficiency category under Innovative Wastewater Technologies.