Turbidity's the cloudiness or haziness of a fluid caused by individual particles (suspended solids) that're generally invisible to the naked eye, similar to smoke in air. The measurement of turbidity's a key test of water quality. Fluids can contain suspended solid matter consisting of particles of many different sizes. While some suspended material will be large enough and heavy enough to settle rapidly to the bottom of the container if a liquid sample's left to stand (the settleable solids), v… (More on Turbidity)