Plain English Meaning
The strongest sideways wind, in knots, that the airplane has been shown to handle safely during takeoff and landing. It comes from the manufacturer's flight tests and is listed in the airplane's handbook.
Full Definition
The highest crosswind component, expressed in knots, at which the manufacturer has demonstrated that a particular airplane can be safely taken off and landed using normal pilot skill and technique. This value is published in the Airplane Flight Manual or Pilot's Operating Handbook and reflects a demonstrated capability during certification, not an absolute structural or aerodynamic limit.