Plain English Meaning
Signed notes from a flight instructor in a student's logbook that say 'this student is approved to fly alone for this specific purpose.' Without the right endorsement, a student pilot is not legally allowed to fly solo.
Full Definition
Written authorizations a flight instructor places in a student pilot's logbook (and, where required, on the student pilot certificate) that grant the student permission to conduct specific solo flight activities under 14 CFR Part 61. Each endorsement is dated, signed by the instructor, and limited in scope — for example, solo flight in a specific make and model, solo cross-country flight, or solo flight to a particular airport. Endorsements expire and must be renewed at intervals defined by the .