How is 'ceiling' defined in aviation terms?

In aviation, 'ceiling' specifically refers to the lowest layer of clouds that is either broken (BKN) or overcast (OVC). This is crucial for pilots as the ceiling affects flight operations and navigational decisions, particularly regarding visual flight rules (VFR) and instrument flight rules (IFR).

When the cloud layer is broken, it means that between 5/8 and 7/8 of the sky is covered with clouds, while overcast indicates that 8/8 of the sky is covered. These conditions determine the minimum altitude at which pilots can operate their aircraft under visual conditions.

The other options such as the highest layer of clouds, cloud coverage percentage, and vertical visibility below overcast do not accurately represent the established definition of 'ceiling' in aviation. The focus on the type of cloud coverage—specifically broken or overcast—provides the necessary information for understanding and applying ceiling in aviation scenarios effectively.