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1. the capacity of a merchant ship expressed in tons, for which purpose a ton is considered as 40 cubic feet of freight or 100 cubic feet of bulk cargo, unless such an amount would weigh more than 2000 pounds in which case the actual weight is used
2. the weight of the cargo of a merchant ship
3. the total amount of shipping of a port or nation, estimated by the capacity of its ships
4. a duty on ships based either on their capacity or their register tonnage
A measure of the size of a ship; it is usually taken to mean gross tonnage or net tonnage, but may also refer to deadweight or displacement tonnage.