Plain English Meaning
A small printed sign in the aircraft that tells you exactly how much weight you can put where, so the airplane stays within its safe loading limits.
Full Definition
A placard installed in an aircraft that shows the approved weight and balance loading limits for that specific airplane, including maximum weights for each seat, baggage compartment, and fuel, along with any restrictions needed to keep the center of gravity within limits.
Context Anchor
Seen in weight-and-balance planning, especially when using the table method shown in the aircraft information or on a placard in the airplane.
Derivation
Placard comes from the French plaquer, meaning to plate or stick on. A placard is literally something fixed in place to information that must not be missed.