The Collaborative International Dictionary of English v.0.48:

Abatement \A*bate"ment\ (-ment), n. [OF. abatement, F.
   abattement.]
   1. The act of abating, or the state of being abated; a
      lessening, diminution, or reduction; removal or putting an
      end to; as, the abatement of a nuisance is the suppression
      thereof.
      [1913 Webster]

   2. The amount abated; that which is taken away by way of
      reduction; deduction; decrease; a rebate or discount
      allowed.
      [1913 Webster]

   3. (Her.) A mark of dishonor on an escutcheon.
      [1913 Webster]

   4. (Law) The entry of a stranger, without right, into a
      freehold after the death of the last possessor, before the
      heir or devisee. --Blackstone.
      [1913 Webster]

   Defense in abatement, Plea in abatement, (Law), plea to
      the effect that from some formal defect (e.g. misnomer,
      lack of jurisdiction) the proceedings should be abated.
      [1913 Webster]

WordNet (r) 3.0 (2006):

abatement
    n 1: an interruption in the intensity or amount of something
         [syn: suspension, respite, reprieve, hiatus,
         abatement]
    2: the act of abating; "laws enforcing noise abatement"