Turbidity is a measure of how well light penetrates water. It is caused by particles (total suspended or dissolved solids) that are generally invisible to the naked eye. The measurement of turbidity is a key test of water quality.
Fluids can contain suspended particles of many different sizes. Some suspended material will be large enough and heavy enough to settle rapidly to the bottom if a liquid sample is left to stand. Very small particles settle very slowly or not at all if the sample is regularly agitated or the particles are colloidal. These small solid particles cause the liquid to appear turbid or murky.