Turbidity is an expression of the optical property of water that causes light to be scattered and absorbed rather than transmitted in straight lines through the sample. Turbidity is caused by suspended and colloidal particulate matter such as clay, silt, finely divided organic and inorganic matter, plankton and other microscopic organisms.

Turbidity is often used for online control of particle filtration processes. The turbidity of feed water to RO/NF should be less than 1 NTU as one of the minimum requirements of feedwater.