Water which is "slightly used," which takes its name from the convention of naming water that is unfit for use "black water." Gray water is water from showers, sinks, dishwashers, and laundry. This specifically excludes toilets and any water used to rinse diapers--any water that contains human waste. Because this water is used to rinse biodegradable bits off of our precious stuff--skin cells, leftovers, body odor, etc.--it's actually not too bad for watering plants with, which would save gallons of "white" potable water. Another novel idea is for each home to have a gray water tank, and to use that for the water in your toilet. Other than the cat having to find a new watering trough, this idea could save 25%-40% of water used in America.
Unfortunately, there are some down sides to a plan where water is used twice for irrigation: fibers (from clothing), sodium (water softener), boron, and many other household chemicals shouldn't be allowed to enter the gray water collection system. It's also unsafe to drink, despite its appearance. Some studies suggest using a vegetable dye to artificially identify it, and to run it through differently colored pipes. Proper disinfection and filtering is required, as well--if you let a spoonful of the wrong chemicals get into your landscape every time you water, they'll work their way up the food chain quickly.