Plain English Meaning
The heaviest the airplane is legally allowed to be when it starts its takeoff roll. Anything heavier than this number is not permitted.
Full Definition
The highest total weight at which an airplane is approved by the manufacturer and certifying authority (such as the FAA) to begin a takeoff roll. This limit is established during aircraft certification testing and is published in the airplane's Type Certificate Data Sheet and Pilot's Operating Handbook. Operating above this weight is prohibited and may compromise structural integrity, performance margins, and controllability.
Context Anchor
Seen during weight-and-balance planning and when checking performance information before a .