The Collaborative International Dictionary of English v.0.48:

Conspiracy \Con*spir"a*cy\, n.; pl. Conspiracies. [See
   Conspiration.]
   1. A combination of people for an evil purpose; an agreement,
      between two or more persons, to commit a crime in concert,
      as treason; a plot.
      [1913 Webster]

            When shapen was all his conspiracy
            From point to point.                  --Chaucer.
      [1913 Webster]

            They made a conspiracy against [Amaziah]. --2 Kings
                                                  xiv. 19.
      [1913 Webster]

            I had forgot that foul conspiracy

            Of the beast Caliban and his confederates. --Shak.
      [1913 Webster]

   2. A concurence or general tendency, as of circumstances, to
      one event, as if by agreement.
      [1913 Webster]

            A conspiracy in all heavenly and earthly things.
                                                  --Sir P.
                                                  Sidney.
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   3. (Law) An agreement, manifesting itself in words or deeds,
      by which two or more persons confederate to do an unlawful
      act, or to use unlawful to do an act which is lawful;
      confederacy.

   Syn: Combination; plot; cabal.
        [1913 Webster]

WordNet (r) 3.0 (2006):

conspiracy
    n 1: a secret agreement between two or more people to perform an
         unlawful act [syn: conspiracy, confederacy]
    2: a plot to carry out some harmful or illegal act (especially a
       political plot) [syn: conspiracy, cabal]
    3: a group of conspirators banded together to achieve some
       harmful or illegal purpose [syn: conspiracy, confederacy]

Moby Thesaurus II by Grady Ward, 1.0:

170 Moby Thesaurus words for "conspiracy":
   Anschluss, accompaniment, accordance, addition, affiliation,
   agglomeration, aggregation, agreement, alliance, amalgamation, art,
   artful dodge, artifice, assimilation, association, blend, blending,
   blind, cabal, cahoots, cartel, centralization, chicane, chicanery,
   co-working, coaction, coalescence, coalition, coincidence,
   collaboration, collectivity, collusion, combination, combine,
   combined effort, combo, complicity, complot, composition, concert,
   concerted action, concomitance, concordance, concourse,
   concurrence, confederacy, confederation, confluence, congeries,
   conglomeration, conjugation, conjunction, conn, connivance,
   connivery, consilience, consolidation, contrivance, contriving,
   cooperation, correspondence, counterplot, coup, covin, craft,
   cute trick, deceit, deep-laid plot, design, device, dirty work,
   disloyalty, dodge, dodgery, ecumenism, embodiment, encompassment,
   engineering, enosis, expedient, faithlessness, fakement, falsity,
   federalization, federation, feint, fetch, finagling, finesse,
   foul play, frame-up, fusion, gambit, game, gimmick, grift, hookup,
   inclusion, incorporation, integration, intrigue, jugglery,
   junction, junta, knavery, league, little game, machination,
   maneuver, maneuvering, manipulation, marriage, meld, melding,
   merger, move, package, package deal, parasitism, perfidiousness,
   perfidy, pettifoggery, pettifogging, plot, plotting, ploy,
   practice, racket, red herring, rigging, ruse, saprophytism, scheme,
   schemery, scheming, sedition, sharp practice, shift, simultaneity,
   skulduggery, sleight, solidification, stratagem, strategy,
   subterfuge, supercherie, symbiosis, synchronism, syncretism,
   syndication, syneresis, synergy, synthesis, tactic, tie-up,
   treacherousness, treachery, treason, trick, trickery,
   underhand dealing, underplot, unification, union, united action,
   web of intrigue, wedding, wile, wily device, wire-pulling



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

CONSPIRACY, crim. law, torts. An agreement between two or more persons to do
an unlawful act, or an act which may become by the combination injurious to
others. Formerly this offence was much more circumscribed in its meaning
than it is now. Lord Coke describes it as "a consultation or agreement
between two or more to appeal or indict an innocent person falsely and
maliciously, whom accordingly they cause to be indicted or appealed and
afterwards the party is acquitted by the verdict of twelve men."
     2. The crime of conspiracy, according to its modern interpretation, may
be of two kinds, Damely, conspiracies against the public, or such as
endanger the public health, violate public morals, insult public justice,
destroy the public peace, or affect public trade or business. See 3 Burr.
1321.
     3. To remedy these evils the guilty persons may be indicted in the name
of the commonwealth. Conspiracies against individuals are such as have a
tendency to injure them in their persons, reputation, or property. The
remedy in these cases is either by indictment or by a civil action.
     4. In order to reader the offence complete, there is no occasion that
any act should be done in pursuance of the unlawful agreement entered into
between the parties, or that any one should have been defrauded or injured
by it. The conspiracy is the gist of the crane. 2 Mass. R. 337; Id. 538 6
Mass. R. 74; 3 S. & R. 220 4 Wend. R. 259; Halst. R. 293 2 Stew. Rep. 360; 5
Harr. & John. 317 8 S. & R. 420. But see 10 Verm. 353.
     5. By the laws of the United State's, St. 1825, c. 76, Sec. 23, 3
Story's L. U. S., 2006, a willful and corrupt conspiracy to cast away, burn
or otherwise destroy any ship or vessel. with intent to injure any
underwriter thereon, or the goods on board thereof, or any lender of money
on such vessel, on bottomry or respondentia, is, by the laws of the United
States, made felony, and the offender punishable by fine not exceeding ten
thousand dollars, and by imprisonment and confinement at hard labor, not
exceeding ten years.
     6. By the Revised Statutes of New York, vol. 2, p. 691, 692, it is
enacted, that if any two or more persons shall conspire, either, 1. To
commit any offence; or, 2. Falsely and maliciously to indict another for any
offence; or, 3. Falsely to move or maintain any suit; or, 4. To cheat and
defraud any person of any property, by any means which are in themselves
criminal; or, 5. To cheat and defraud any person of any property, by means
which, if executed, would amount to a cheat, or to obtaining property by
false pretences; or, 6. To commit any act injurious to the public health, to
public morals, or to trade and commerce, or for the perversion or
obstruction of justice, or the due administration of the laws; they shall be
deemed guilty of a misdemeanor. No other conspiracies are there punishable
criminally. And no agreement, except to commit a felony upon the person of
another, or to commit arson or burglary, shall be deemed a conspiracy,
unless some act besides such agreement be done to effect the object thereof,
by one or more of the parties to such agreement.
     7. When a felony has been committed in pursuance of a conspiracy, the
latter, which is only a misdemeanor, is merged in the former; but when a
misdemeanor only has been committed in pursuance of such conspiracy, the two
crimes being of equal degree, there can be no legal technical merger. 4
Wend. R. 265. Vide 1 Hawk. 444 to 454; 3 Chit. Cr. Law, 1138 to 1193 3 Inst.
143 Com. Dig. Justices of the Peace, B 107; Burn's Justice, Conspiracy;
Williams' Justice, Conspiracy; 4 Chit. Blacks. 92; Dick. Justice Conspiracy,
Bac. Ab. Actions on the Case, G 2 Russ. on Cr. 553 to 574 2 Mass. 329 Id.
536 5 Mass. 106 2 D R. 205; Whart. Dig. Conspiracy; 3 Serg. & Rawle, 220; 7
Serg. & Rawle, 469 4 Halst. R. 293; 5 Harr. & Johns. 317 4 Wend. 229; 2
Stew. R. 360;1 Saund. 230, u. 4. For the French law, see Merl. Rep. mot
Conspiration Code Penal, art. 89.