From AMS Glossary
A layer of oil on the water surface, usually as a result of an accidental or deliberate spill of crude oil or other petroleum product.
The thickness of an oil slick can range from a few molecules thick up to many millimeters. Thin oil slicks appear as a "blue sheen", due to optical interference effects. Even when very thin, slicks cause significant damping of centimeter-scale surface waves and thus appear as low-backscatter regions in airborne or satellite radar images. After spillage, oil slicks spread by the effects of the winds, tides, gravity, surface tension, and ocean current shear and turbulence. They eventually disperse by evaporation, dissolution, and downmixing by breaking waves. Wave action can also lead to the formation of oil-in-water emulsions.