1. A measure of the degree of transparency of a fluid: The turbidity of drinking water is important to make sure of its purity.
2. Muddiness created by stirring up sediments or having foreign particles suspended: Turbidity is brought about by foreign particles mixed up or floating around within a liquid.
4. A measure of the opacity of the atmosphere: A perfectly clear sky has a turbidity of "0", and a completely opaque sky has a turbidity of "1".

Haziness in the atmosphere due to aerosols, such as dust: When atmospheric turbidity is zero, the sky has no dust, as measured by a sun photometer, an instrument that measures luminous intensity or brightness, luminous flux, light distribution, color, etc.