Glossary
What is a ceiling in an aviation weather report?
The height above the ground of the lowest cloud layer reported as broken or overcast, or the vertical visibility into an obscuration such as fog. Layers reported as few or scattered are not ceilings, however low they sit.
That is the whole entry for now. The longer version, with the mechanics and worked examples, is still to be written. See autorotation for how those read once they exist.