Plain English Meaning
Flying the aircraft alone, with no instructor or other person on board, after your instructor has signed you off as ready.
Full Definition
A flight in which the student pilot is the sole occupant of the aircraft, conducted under the authorization and supervisory endorsement of an authorized flight instructor.
Context Anchor
Seen in student pilot training when discussing the first flights made without an instructor in the airplane, including the skills that must be safe before that point.
Derivation
From the Italian solo, meaning 'alone,' originally a musical term for a passage performed by one player. In aviation, it carries the same idea: one person, performing the task on their own.
Why Pilots Care
It marks a training milestone confirming the student can handle the aircraft safely without supervision.