Plain English Meaning
Civilian flying that isn't part of an airline schedule and isn't military. If it's a private pilot, a flight school, a corporate jet, a crop duster, or a charter flight, it's general aviation.
Full Definition
All civil aviation operations other than scheduled air carrier service and military operations. This includes private flying, flight training, business and corporate flights, agricultural flying, charter operations, aerial survey, banner towing, and most other non-airline civilian flight activity.
Context Anchor
Seen in FAA publications, safety material, airport planning, accident reports, and discussions that separate small-aircraft and non-airline flying from airline operations.
Derivation
The word 'general' here means 'covering everything else.' Once you set aside airlines and the military, what's is the general — meaning broad and varied — of civil flying.