Turbidity is how cloudy or muddy a liquid is — how much tiny stuff is floating in it so you cannot see through. Clear water has low turbidity. Water after a typhoon, full of mud and sand, has high turbidity. The important thing is that the stuff is *floating*, not sitting at the bottom; once it settles, the water gets clear again and the turbidity drops. This is a measuring word, so it is mostly used by scientists, water companies, and people who test rivers and fish tanks. They give it a number with a machine that shines light through the water: the more light gets blocked, the higher the number. Normal people do not say this word; they say 'the water is cloudy' or 'the water is dirty'. You will meet 'turbidity' in a news report about tap water after heavy rain, in a science class, or on a report about a reservoir. The adjective 'turbid' is even rarer in speech — writers sometimes use it for a river, and sometimes, in a book-ish way, for feelings or thoughts that are confused and unsettled, though actually that use sounds old-fashioned now.