The Collaborative International Dictionary of English v.0.48:

Ton \Ton\ (t[u^]n), n. [OE. tonne, tunne, a tun, AS. tunne a
   tun, tub, a large vessel; akin to G. & F. tonne a ton, tun,
   LL. tunna a tun; all perhaps of Celtic origin; cf. Ir. &
   Gael. tunna a tun. Cf. Tun,Tunnel.] (Com.)
   A measure of weight or quantity. Specifically:
   [1913 Webster]
   (a) The weight of twenty hundredweight.
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   Note: In England, the ton is 2,240 pounds. In the United
         States the ton is commonly estimated at 2,000 pounds,
         this being sometimes called the short ton, while that
         of 2,240 pounds is called the long ton.
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   (b) (Naut. & Com.) Forty cubic feet of space, being the unit
       of measurement of the burden, or carrying capacity, of a
       vessel; as a vessel of 300 tons burden. See the Note
       under Tonnage.
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   (c) (Naut. & Com.) A certain weight or quantity of
       merchandise, with reference to transportation as freight;
       as, six hundred weight of ship bread in casks, seven
       hundred weight in bags, eight hundred weight in bulk; ten
       bushels of potatoes; eight sacks, or ten barrels, of
       flour; forty cubic feet of rough, or fifty cubic feet of
       hewn, timber, etc.
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   Note: Ton and tun have the same etymology, and were formerly
         used interchangeably; but now ton generally designates
         the weight, and tun the cask. See Tun.
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The Collaborative International Dictionary of English v.0.48:

Ton \Ton\ (t[o^]n), obs.
   pl. of Toe. --Chaucer.
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The Collaborative International Dictionary of English v.0.48:

Ton \Ton\ (t[u^]n), n. [Cf. Tunny.] (Zool.)
   The common tunny, or horse mackerel.
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The Collaborative International Dictionary of English v.0.48:

Ton \Ton\ (t[^o]n), n. [F. See Tone.]
   The prevailing fashion or mode; vogue; as, things of ton.
   --Byron.
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         If our people of ton are selfish, at any rate they show
         they are selfish.                        --Thackeray.
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   Bon ton. See in the Vocabulary.
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WordNet (r) 3.0 (2006):

ton
    n 1: a United States unit of weight equivalent to 2000 pounds
         [syn: short ton, ton, net ton]
    2: a British unit of weight equivalent to 2240 pounds [syn:
       long ton, ton, gross ton]

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

TON. Twenty hundred weight, each hundred weight being one hundred and twelve
pounds avoirdupois. See act of congress of Aug. 30, 1842, c. 270, s. 20.