Source : Webster's Revised Unabridged Dictionary (1913)

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

   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) .

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

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

   4. The whole amount of shipping estimated by tons; as, the
      tonnage of the United States. See {Ton}.

   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.

Source : WordNet®

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