Plain English Meaning
A written test the instructor gives a student before letting them fly alone for the first time. It checks that the student knows the rules and the aircraft well enough to be safe without an instructor on board.
Full Definition
A written test that a student pilot must complete and have reviewed by their authorized instructor before being endorsed for solo flight, covering applicable regulations, airspace rules, and the flight characteristics and operating limitations of the make and model of aircraft to be flown solo. It is required by 14 CFR 61.87 and is administered and graded by the instructor, who then reviews any incorrect answers with the student before issuing the solo endorsement.
Context Anchor
You will encounter this term during student pilot training, especially before the first solo flight and when discussing instructor endorsements.