The secchi disc is used to measure the turbidity of water. It is a simple device consisting of a circular disk, typically white and black on opposite sides, that is lowered into the water until it disappears from view. The depth at which the disk disappears from view, known as the Secchi depth, is used as a measure of the water's clarity or turbidity. Turbidity refers to the degree to which the water is cloudy or hazy due to suspended particles such as sediment or plankton.