Plain English Meaning
The official rules that say what the airplane is allowed to do and what it must never do. They cover things like how fast you can fly, how heavy you can load it, what fuel to use, and what equipment must be working.
Full Definition
The set of approved boundaries within which an aircraft may legally and safely be operated, covering items such as airspeed limits, weight and balance limits, powerplant limits, maneuvers permitted, fuel and oil specifications, and required equipment. For most modern certificated airplanes, these limitations are found in Section 2 of the FAA-approved Airplane Flight Manual or Pilot's Operating Handbook, and are reinforced by markings, placards, and in the .