What is defined as the lowest BKN or OVC layer?

The correct answer is the definition of "ceiling," which refers to the height of the lowest layer of broken (BKN) or overcast (OVC) clouds. In aviation and meteorological contexts, ceiling is a crucial term used to assess flight safety and navigational conditions. It signifies the vertical visibility above the ground, important for pilots during takeoff and landing. When the clouds are broken or overcast, determining the ceiling helps in understanding how much of the sky is obscured and what visual limits might be encountered.

Visibility, while integral to understanding weather conditions, relates to how far one can see horizontally, rather than the vertical measurement of cloud layers. Cloud base focuses specifically on the height of the lowest cloud layer but does not encompass the overall condition regarding BKN or OVC classifications. Sky cover is a broader term that addresses the amount of the sky covered by clouds but does not specifically mention the height of those clouds. Thus, "ceiling" is the most precise term for describing the lowest significant cloud layer impacting flight operations.