The Collaborative International Dictionary of English v.0.48:

Set-off \Set"-off`\, n. [Set + off.]
   1. That which is set off against another thing; an offset.
      [1913 Webster]

            I do not contemplate such a heroine as a set-off to
            the many sins imputed to me as committed against
            woman.                                --D. Jerrold.
      [1913 Webster]

   2. That which is used to improve the appearance of anything;
      a decoration; an ornament.
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   3. (Law) A counterclaim; a cross debt or demand; a distinct
      claim filed or set up by the defendant against the
      plaintiff's demand.
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   Note: Set-off differs from recoupment, as the latter
         generally grows out of the same matter or contract with
         the plaintiff's claim, while the former grows out of
         distinct matter, and does not of itself deny the
         justice of the plaintiff's demand. Offset is sometimes
         improperly used for the legal term set-off. See
         Recoupment.
         [1913 Webster]

   4. (Arch.) Same as Offset, n., 4.
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   5. (Print.) See Offset, 7.
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   Syn: Set-off, Offset.

   Usage: Offset originally denoted that which branches off or
          projects, as a shoot from a tree, but the term has
          long been used in America in the sense of set-off.
          This use is beginning to obtain in England; though
          Macaulay uses set-off, and so, perhaps, do a majority
          of English writers.
          [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Offset \Off"set`\, n. [Off + set. Cf. Set-off.]
   In general, that which is set off, from, before, or against,
   something; as:
   [1913 Webster]

   1. (Bot.) A short prostrate shoot, which takes root and
      produces a tuft of leaves, etc. See Illust. of
      Houseleek.
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   2. A sum, account, or value set off against another sum or
      account, as an equivalent; hence, anything which is given
      in exchange or retaliation; a set-off.
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   3. A spur from a range of hills or mountains.
      [1913 Webster]

   4. (Arch.) A horizontal ledge on the face of a wall, formed
      by a diminution of its thickness, or by the weathering or
      upper surface of a part built out from it; -- called also
      set-off.
      [1913 Webster]

   5. (Surv.) A short distance measured at right angles from a
      line actually run to some point in an irregular boundary,
      or to some object.
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   6. (Mech.) An abrupt bend in an object, as a rod, by which
      one part is turned aside out of line, but nearly parallel,
      with the rest; the part thus bent aside.
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   7. (Print.) A more or less distinct transfer of a printed
      page or picture to the opposite page, when the pages are
      pressed together before the ink is dry or when it is poor;
      an unitended transfer of an image from one page to
      another; called also setoff.
      [1913 Webster +PJC]

   8. See offset printing.
      [PJC]

   Offset staff (Surv.), a rod, usually ten links long, used
      in measuring offsets.
      [1913 Webster]

WordNet (r) 3.0 (2006):

setoff
    n 1: structure where a wall or building narrows abruptly [syn:
         set-back, setoff, offset]

Moby Thesaurus II by Grady Ward, 1.0:

87 Moby Thesaurus words for "setoff":
   abatement, agio, allowance, antipode, antipodes, antipole,
   antithesis, antonym, balance, ballast, bank discount, breakage,
   cash discount, chain discount, charge-off, concession,
   consideration, contra, converse, counter, counterbalance,
   countercheck, counterpoint, counterpoise, counterpole, counterterm,
   counterweight, cut, deduction, depreciation, discount, drawback,
   equipoise, equivalent, foil, give-and-take, impress, impression,
   imprint, inverse, kickback, letterpress, mackle, makeweight,
   obverse, offcut, offprint, offset, opposite, opposite number,
   outset, outsetting, outstart, penalty, penalty clause, percentage,
   premium, price reduction, price-cut, print, quid pro quo, rebate,
   rebatement, reduction, refund, reissue, reprint, reverse, rollback,
   salvage, setout, something of value, stamp, start, start-off,
   starting, takeoff, tare, the contrary, the other side,
   time discount, tit for tat, trade discount, tret, underselling,
   vis-a-vis, write-off



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

SET-OFF, contracts, practice. Defalcation; (q.v.) a demand which a defendant
makes against the plaintiff in the suit for the purpose of liquidating the
whole or a part of his claim.
     2. A set-off was unknown to the common law, according to which mutual
debts were distinct and inextinguishable except by actual payment or
release. 1 Rawle's R. 293; Babb. on Set-off, 1.
     3. The statute 2 Geo. II., c. 22, which has been generally adopted in
the United States with some modifications however, allowed, in cases of
mutual debts, the defendant to set his debt against the other, either by
pleading it in bar, or giving it in evidence, when proper notice had been
given of such intention, under the general issue. The statute being made for
the benefit of the defendant, is not compulsory; 8 Watts, R. 39; the
defendant may Waive his right, and bring a cross action against the
plaintiff. 2 Campb. 594; 5 Taunt. 148; 9 Watts, R. 179
     4. It seems, however, that in some cases of intestate estates, and of
insolvent estates, perhaps owing to the peculiar wording of the law, the
statute has been held to operate on the rights of the parties before action
brought, or an act done by either of them. 2 Rawle's R. 293; 3 Binn. Rep.
135; Bac. Ab. Bankrupt K.
     5. Set-off takes place only in actions on contracts for the payment of
money, as assumpsit, debt and covenant. A set-off is not allowed in actions
arising ex delicto, as, upon the case, trespass, replevin or detinue. Bull.
N. P. 181.
     6. The matters which may be set off, may be mutual liquidated debts or
damages, but unliquidated damages cannot be set off. 1 Black. R. 394; 2
John. 150; 8 Conn. 325; 1 McCord, 7; 3 Wend. 400; 1 Stew. & Port. 19; 2
Yeates, 208; 1 Sumn. 471; 2 Blackf. 31; 1 A. K. Marsh. 41; 6 Halst. 397; 5
Wash. C. C. 232 3 Bibb, 49; 2 Caines, 33. The statutes refer only to mutual
unconnected debts; for at common law, when the nature of the employment,
transaction or dealings necessarily constitute an account consisting of
receipts and payments, debts and credits, the balance only is considered to
be the debt, and therefore in an action, it is not necessary in such cases
either to plead or give notice of set-off. 4 Burr. 2221.
     7. In general, when the government is plaintiff, no set-off will be
allowed. 9 Pet. 319; 4 Dall. 303. See 9 Cranch, 313; Paine, 156. But when an
act of congress authorizes such set-off, it may be made. 9 Cranch, 213.
     8. Judgments in the same rights may be set off against each other at
the discretion of the court. 3 Bibb 233; 3 Watts 78; 3 Halst. 172; 4 Hamm.
90; 1 Stew. & Port. 24; 7 Mass. 140, 144; 8 Cowen 126. Vide Compensation;
also Montagu on Set-off; Babington on Set-off; 3 Stark. Ev. h.t.; Amer. Dig.
h.t.; Whart. Dig. h.t.; 3 Chit. Bl. Com. 304, n.; 1 Chit. Pl. Index, h.t.; 8
Vin. Ab. 556; Bac. Ab. h.t. 1 Sell. Pr. 321; 5 Com. Dig. 595; 6 Id. 335; 7
Id. 336; 8 Id. 927; Chit. Pr. Index, h.t.; Bouv. Inst. Index, h.t. Vide
Factor.