The Collaborative International Dictionary of English v.0.48:

Burden \Bur"den\ (b[^u]"d'n), n. [Written also burthen.] [OE.
   burden, burthen, birthen, birden, AS. byr[eth]en; akin to
   Icel. byr[eth]i, Dan. byrde, Sw. b["o]rda, G. b["u]rde, OHG.
   burdi, Goth. ba['u]r[thorn]ei, fr. the root of E. bear, AS.
   beran, Goth. bairan. [root]92. See 1st Bear.]
   1. That which is borne or carried; a load.
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            Plants with goodly burden bowing.     --Shak.
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   2. That which is borne with labor or difficulty; that which
      is grievous, wearisome, or oppressive.
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            Deaf, giddy, helpless, left alone,
            To all my friends a burden grown.     --Swift.
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   3. The capacity of a vessel, or the weight of cargo that she
      will carry; as, a ship of a hundred tons burden.
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   4. (Mining) The tops or heads of stream-work which lie over
      the stream of tin.
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   5. (Metal.) The proportion of ore and flux to fuel, in the
      charge of a blast furnace. --Raymond.
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   6. A fixed quantity of certain commodities; as, a burden of
      gad steel, 120 pounds.
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   7. A birth. [Obs. & R.] --Shak.
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   Beast of burden, an animal employed in carrying burdens.

   Burden of proof [L. onus probandi] (Law), the duty of
      proving a particular position in a court of law, a failure
      in the performance of which duty calls for judgment
      against the party on whom the duty is imposed.
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   Syn: Burden, Load.

   Usage: A burden is, in the literal sense, a weight to be
          borne; a load is something laid upon us to be carried.
          Hence, when used figuratively, there is usually a
          difference between the two words. Our burdens may be
          of such a nature that we feel bound to bear them
          cheerfully or without complaint. They may arise from
          the nature of our situation; they may be allotments of
          Providence; they may be the consequences of our
          errors. What is upon us, as a load, we commonly carry
          with greater reluctance or sense of oppression. Men
          often find the charge of their own families to be a
          burden; but if to this be added a load of care for
          others, the pressure is usually serve and irksome.
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WordNet (r) 3.0 (2006):

burden of proof
    n 1: the duty of proving a disputed charge