Source : Webster's Revised Unabridged Dictionary (1913)

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

            When shapen was all his conspiracy From point to
            point.                                --Chaucer.

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

            I had forgot that foul conspiracy

            Of the beast Caliban and his confederates. --Shak.

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

            A conspiracy in all heavenly and earthly things.
                                                  --Sir P.
                                                  Sidney.

   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.

Source : WordNet®

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