Glossary
What is a transponder?
A radio unit that replies to ground radar with the aircraft's identity and altitude. Most ADS-B Out installations are built around an existing transponder, adding the broadcast function to it, and most aircraft flying in controlled airspace carry one regardless of ADS-B equipage.
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.