What is Tonnage?

Legal Definition
Tonnage is a measure of the cargo-carrying capacity of a ship. The term derives from the taxation paid on tuns or casks of wine, and was later used in reference to the weight of a ship's cargo; however, in modern maritime usage, "tonnage" specifically refers to a calculation of the volume or cargo volume of a ship. Tonnage should not be confused with displacement, which refers to the actual weight of the vessel.

Measurement of tonnage can be complicated, not least because it is used to assess fees on commercial shipping.
-- Wikipedia
Legal Definition
Mar. law. The capacity of a ship or vessel.

2. The act of congress of March 2, 1799, s. 64, 1 Story's L. U. S. 630, directs that to ascertain the tonnage of any ship or vessel, the surveyor, &c. shall, if the said ship or vessel be double decked, take the length thereof from the forepart of the main stem, to the afterpart of the stern post, above the upper deck, the breadth thereof, at the broadest part above the mainwales, half of which breadth shall be accounted the depth of such vessel, and then deduct from the length three-fifths of the breadth, multiply the remainder by the breadth and the product of the depth, and shall divide this last product by ninety-five, the quotients whereof shall be deemed the true contents or tonnage of such ship or vessel. And if such ship or vessel shall be single decked, the said, surveyor shall take the length and breadth as above directed, in respect to a double deck ship or vessel, and shall deduct from the length three-fifths of the breadth, and taking the depth from the under-side of the deck plank to the ceiling of the hold, shall multiply and divide as aforesaid, and the quotient shall be deemed the tonnage of such ship or vessel.

3. The duties paid on the tonnage of a ship or vessel are also called tonnage.

4. These duties are altogether abolished in relation. to American vessels by the act of May 31, 1830, s. 1, 4 Story's Laws U. S. 2216. And by the second section of the same act, all tonnage duties on foreign vessels are abolished, provided the president of the, United States shall be satisfied that the discriminating or countervailing duties of such foreign nation, so far as they operate to the disadvantage. of the United States, have been abolished.

5. The constitution of the United States provides, art. 1, s. 10, n. 2, that no state shall, without the consent of congress, lay any duty on tonnage.
-- Bouviers Law Dictionary
Legal Definition
The capacity of a vessel for carrying freight or other loads, calculated in tons. But the way of estimating the tonnage varies in different countries. In England, tonnage denotes the actual weight in tons which the vessel can safely carry; in America, her carrying capacity estimated from the cubic dimensions of the hold. See Roberts v. Opdyke, 40 N. Y. 259. The "tonnage" of a vessel is her capacity to carry cargo, and a charter of "the whole tonnage" of a ship transfers to the charterer only the space necessary for that purpose. Timing v. Insurance Co... 103 Mass. 405, 4 Am. Rep. 567. The tonnage of a vessel is her intemni cubical capacity, in tons. Inman S. S. Co. v. Tinker, 94 U. S. 238, 24 L. Ed. 118.
-- Black's Law Dictionary
Legal Definition
A ship’s carrying capacity designated in tons. See 130 N. Y. 160, 29 N. E. 137.
-- Ballentine's Law Dictionary