Plain English Meaning
Flying the aircraft by yourself, with no instructor and no passengers, once your instructor has signed you off as ready.
Full Definition
A flight conducted by a student pilot alone in the aircraft, without an instructor or any other person on board, after being endorsed by an authorized instructor as competent and safe to do so.
Context Anchor
Seen in student pilot training, especially when discussing takeoffs, climbs, landings, and other maneuvers performed without an instructor on board.
Derivation
From the Italian 'solo' meaning 'alone,' originally a musical term for a passage performed by one person. In aviation it carries the same idea: one person, operating the aircraft without help.
Why Pilots Care
Solo flight marks a required training milestone confirming the student can manage the aircraft, emergencies, and without supervision.