I suppose that the technical defination would be activities that are regulated under FAR
Part 91 or Part 135 regulations and fall under one or more of the following classifications:
Non-military flight training (commercial flight schools)
Corporate Flight Department operations (Business Aviation)
Operation of registered experimantal, heavy, medium, small, light, ultra-light and lighter-than-air aircraft (balloons)
Aircraft operated under FAR
Part 135 fall into the General Aviation classification as they operate for hire, but are not required to meet the same requirements as aircraft operated by scheduled carriers under Part 121. Also, many aircraft operating under Part 135 do not do so exclusively, being operated by their owners both for private/business purposes in addition to air taxi work.
In many cases, the only physical difference between an aircraft operated under Part 91 and a similar type operated under Part 135 is the cabin appointments. Our family's Cessna 414 is a good example of this, as anyone who's been around the Cessna 300/400 types can tell that it was a former Part 135 aircraft at one time in its life. Two details of the interior appointments point that out immediately. First, the aircraft has a partition between the flight deck and the passenger cabin. Most Cessna 300/400 series airframes that were delivered to Part 91 operators do not have the partition. The second clue is the difference in upholstery of the seats on the flight deck and the passenger cabin seats. The flight deck seats are battleship grey fabric while the seats in the rear cabin (except the belted toilet) are navy blue leather. Add the presence of all the optional features such as fold out tables, cup holders, passenger display (airspeed, altitude and outside temperature) and the aircraft's Part 135 background is immediately noticable.
"When a man runs on railroads over half of his lifetime he is fit for nothing else-and at times he don't know that."