What does the term 'cloud ceiling' refer to in aviation?

In aviation, the term 'cloud ceiling' specifically refers to the height, above ground level, of the lowest layer of clouds that is reported as being significant. This is crucial for pilots and air traffic control as it affects flight operations, particularly those involving visual flight rules (VFR). When the ceiling is low, it may restrict visibility below that level, making it difficult for pilots to navigate visually.

In contrast, the other options provided describe different aspects of cloud and weather phenomena but do not accurately define cloud ceiling. The height above sea level of the highest cloud pertains more to overall cloud formations rather than the ceiling itself. Density altitude, which relates to atmospheric conditions and aircraft performance, does not address cloud height. Lastly, vertical visibility through clouds refers to how far one can see within a cloud rather than the height of clouds above the ground, further distinguishing it from the definition of cloud ceiling.