Secchi disk
noun
an opaque, white or black-and-white disk used to measure the cloudiness or turbidity of ocean water by the point at which it is no longer visible from the surface.
Scientific definitions for secchi disk
Secchi disk
An instrument used for measuring the clarity of water, especially seawater. It consists of a circular plate divided into alternating black and white quadrants and attached to a long measuring tape. The plate is lowered into the water, and the depth at which it is no longer visible from the surface is recorded. The Secchi disk is named after its inventor, Italian astronomer Angelo Secchi (1818-1878). See also Secchi depth.