Plain English Meaning
The heaviest the aircraft is allowed to be when it starts the takeoff roll, given today's runway, weather, and obstacles. The lowest weight that any single rule produces is the one you must respect.
Full Definition
The highest weight at which an aircraft is permitted to begin the takeoff roll on a given runway under the existing conditions. It is determined by the most limiting of several factors: structural takeoff weight limits, runway length available, climb performance requirements (including one-engine-inoperative obstacle clearance), tire speed and brake energy limits, and field elevation, , , and .