Gray water is water that has first been used in the house and then is reused outside for plants and trees. It is water that comes from bathroom sinks and showers, as well as from dishwashers and washing machines. This does not include water that is from the bathroom toilet because this water contains fecal matter that has the potential of containing hazardous bacterial organisms. Also, any water from the washing machine or sink that has been used to wash diapers is not allowed to be used, due to the bacteria from human waste.
Via Bert Guevara