Impairment of a medium's transparency by the scatter radiation and absorption caused by optically denser inclusions, such as particles.
Turbidity is an everyday phenomenon. We refer to it with attributes like: poor transparency, contrast reduction, lateral luminosity, etc. What is responsible for all of these effects is the optical phenomenon of light scatter. The most beautiful examples of scattered light are the blue of the sky and the red of the sunset.
If a liquid or gaseous carrier medium contains foreign matter, it will occur either dissolved or in the form of suspended particles. In the case of a solution the smallest objects are molecules (or ions); in a suspension, the smallest are the individual particles - each consisting of millions of molecules. The difference between solutions and suspensions is mainly a matter of particle size. The border between them is fuzzy and varies depending on which author you read.
From the standpoint of process technology, the important question is whether the foreign substance is filterable or not. Suspensions that do not lend themselves to filtration are called colloidal solutions. The following Figure illustrates the detection limits with various techniques and the particle size range of certain substances.