secchi depth

noun

1. the depth at which a Secchi disk is no longer visible from the surface of ocean water.

1. A measure of the clarity of water, especially seawater. Secchi depth is measured using a circular plate, known as a Secchi disk, which is lowered into the water until it is no longer visible. High Secchi depths indicate clear water; whereas low Secchi depths indicate cloudy or turbid water. See also Secchi disk.