Glossary
What is Class D airspace?
Controlled airspace around an airport that has an operating control tower but no radar service requirement. Entry requires two-way radio communication established with the tower. It generally exists only while the tower is open and reverts to another class of airspace once it closes.
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.