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.
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            They made a conspiracy against [Amaziah]. --2 Kings
                                                  xiv. 19.
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            I had forgot that foul conspiracy

            Of the beast Caliban and his confederates. --Shak.
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   2. A concurence or general tendency, as of circumstances, to
      one event, as if by agreement.
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            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]