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]

Moby Thesaurus II by Grady Ward, 1.0:

113 Moby Thesaurus words for "abatement":
   abridgment, agio, allayment, alleviation, allowance, analgesia,
   anesthesia, anesthetizing, appeasement, assuagement, attenuation,
   attrition, bank discount, blunting, breakage, calming,
   cash discount, chain discount, charge-off, concession, contraction,
   cut, dampening, damping, deadening, debilitation, decrease,
   decrement, decrescence, deduction, deflation, demulsion,
   depreciation, depression, devitalization, dilution, diminishment,
   diminution, discount, drawback, dulcification, dulling, dying,
   dying off, ease, easement, easing, effemination, enervation,
   enfeeblement, evisceration, exhaustion, extenuation, fade-out,
   falling-off, fatigue, hushing, inanition, kickback, languishment,
   leniency, lessening, letdown, letup, lightening, loosening,
   lowering, lulling, miniaturization, mitigation, modulation,
   mollification, numbing, pacification, palliation, penalty,
   penalty clause, percentage, premium, price reduction, price-cut,
   quietening, quieting, rebate, rebatement, reduction, refund,
   relaxation, relief, remedy, remission, rollback, sagging, salvage,
   salving, scaling down, setoff, simplicity, slackening, softening,
   soothing, subduement, subtraction, tare, tempering, thinning,
   time discount, trade discount, tranquilization, tret, underselling,
   weakening, write-off