A sea surface film in which organic accumulation exceeds a threshold such that it becomes visible as a slick, i.e. a sea surface feature that appears as smooth grey spots or stripes in contrast to the surrounding deep blue waters. The smoothing effect is due to the accumulation at the sea–air interface of organic compounds, many of them surface–active, which enhance solar reflection at the surface by damping the capillary waves. Slicks are thought to play a significant role in heat flux and gas exchange, biogeochemical cycles, and pollutant dispersion dynamics as a consequence of the organic enrichment and their location at the boundary between atmosphere and ocean.