Glossary
What is an endorsement?
A signed and dated entry an instructor makes in a pilot's logbook approving them for something specific: solo flight, a knowledge test, a cross country, a particular kind of airplane. It lives in the logbook rather than on the certificate.
Also called Instructor endorsement
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.