noun.
A principal medium-level cloud type, white and/or gray in color, which occurs as a layer or patch with a waved aspect, the elements of which appear as laminae, rounded masses, or rolls. These elements usually are sharply outlined, but they may become partly fibrous or diffuse; they may or may not be merged; they generally have shadowed parts; and, by convection, when observed at an angle of more than 30° above the horizon, subtend an angle between 1 degrees and 5 degrees. (Abbreviated Ac.)