Ship's mass when launched, empty: hull, machinery, equipment, no fuel, water, ballast, stores, or cargo.
Regulatory detail & full definition
Established at the inclining experiment after launch. Used for scrap valuation (LWT × scrap-steel price), and as the constant in displacement = LWT + DWT. Some yards over-build slightly to ensure the ship meets contract DWT after the actual LWT comes in heavier than calculated.