What does "gray water" refer to?

Gray water refers specifically to used water that has been generated from various household activities such as washing dishes, bathing, and laundry. This type of water is distinct from "black water," which is wastewater from toilets and contains harmful pathogens and organic matter. Gray water can still contain some level of contamination but is generally considered less harmful and can often be reused for irrigation, flushing toilets, or other non-potable purposes, provided that it meets local regulations.

The other options describe different types of water that do not fall under the definition of gray water. Freshwater sources like rivers and lakes are not categorized as gray water since they are not used or contaminated. Treated and purified water refers to water that has undergone processing to remove contaminants for safe consumption, which is significantly different from the wastewater generated in daily activities. Rainwater collection also describes a specific practice of gathering precipitation for reuse, and is not classified as gray water.