The Collaborative International Dictionary of English v.0.48:

Lump \Lump\ (l[u^]mp), n. [Cf. OD. lompe piece, mass. Cf.
   Lunch.]
   1. A small mass of matter of irregular shape; an irregular or
      shapeless mass; as, a lump of coal; a lump of iron ore. "
      A lump of cheese." --Piers Plowman. " This lump of clay."
      --Shak.
      [1913 Webster]

   2. A mass or aggregation of things.
      [1913 Webster]

   3. (Firearms) A projection beneath the breech end of a gun
      barrel.
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   In the lump,

   In a lump, the whole together; in gross.
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            They may buy them in the lump.        --Addison.

   Lump coal, coal in large lumps; -- the largest size brought
      from the mine.

   Lump sum,
      (a) a gross sum without a specification of items; as, to
          award a lump sum in satisfaction of all claims and
          damages.
      (b) a single sum paid once in satisfaction of a claim, as
          contrasted with the alternate choice of several
          payments over a period of time; -- sometimes allowed,
          e.g., as an alternative to periodical pension payments
          for a lifetime.
          [1913 Webster]

WordNet (r) 3.0 (2006):

lump sum
    n 1: a complete payment consisting of a single sum of money