The Collaborative International Dictionary of English v.0.48:

Tonnage \Ton"nage\ (?; 48), n. [From Ton a measure.]
   [1913 Webster]
   1. The weight of goods carried in a boat or a ship.
      [1913 Webster]

   2. The cubical content or burden of a vessel, or vessels, in
      tons; or, the amount of weight which one or several
      vessels may carry. See Ton, n.
      (b) .
          [1913 Webster]

                A fleet . . . with an aggregate tonnage of
                60,000 seemed sufficient to conquer the world.
                                                  --Motley.
          [1913 Webster]

   3. A duty or impost on vessels, estimated per ton, or, a
      duty, toll, or rate payable on goods per ton transported
      on canals.
      [1913 Webster]

   4. The whole amount of shipping estimated by tons; as, the
      tonnage of the United States. See Ton.
      [1913 Webster]

   Note: There are in common use the following terms relating to
         tonnage: (a) Displacement. (b) Register tonnage, gross
         and net. (c) Freight tonnage. (d) Builders'
         measurement. (e) Yacht measurement. The first is mainly
         used for war vessels, where the total weight is likely
         to be nearly constant. The second is the most
         important, being that used for commercial purposes. The
         third and fourth are different rules for ascertaining
         the actual burden-carrying power of a vessel, and the
         fifth is for the proper classification of pleasure
         craft. Gross tonnage expresses the total cubical
         interior of a vessel; net tonnage, the cubical space
         actually available for freight-carrying purposes. Rules
         for ascertaining these measurements are established by
         law.
         [1913 Webster]

WordNet (r) 3.0 (2006):

tonnage
    n 1: a tax imposed on ships that enter the US; based on the
         tonnage of the ship [syn: tonnage, tunnage, tonnage
         duty]

Moby Thesaurus II by Grady Ward, 1.0:

53 Moby Thesaurus words for "tonnage":
   accommodation, argosy, avoirdupois, beef, beefiness, bottoms,
   burden, capacity, carriage, cartage, content, cordage, deadweight,
   drayage, expressage, fatness, fleet, flotilla, freight, freightage,
   gravity, gross weight, haulage, heaviness, heft, heftiness, limit,
   line, liveweight, measure, merchant fleet, merchant navy, navy,
   neat weight, net, net weight, overbalance, overweight,
   ponderability, ponderosity, ponderousness, poundage, quantity,
   room, shipping, ships, space, stowage, underweight, volume, weight,
   weightiness, whaling fleet



Bouvier's Law Dictionary, Revised 6th Ed (1856):

TONNAGE, 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 underside 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.