a thin film of oil floating on top of water (especially crude oil spilled from a ship)
glossy smooth spot on the water's surface (created by an oil spill, environmental pollution)
An oil slick is a layer of oil that is floating on the sea or on a lake because it has accidentally come out of a ship or container. The oil slick is now 35 miles long. A layer of oil floating on the surface of water. a large area of oil floating on the surface of the sea or a river = slick