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.
      [1913 Webster]

   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]

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