The Collaborative International Dictionary of English v.0.48:

Iconoclast \I*con"o*clast\, n. [Gr. e'ikw`n image + ? to break:
   cf. F. iconoclaste.]
   1. A breaker or destroyer of images or idols; a determined
      enemy of idol worship.
      [1913 Webster]

   2. One who exposes or destroys impositions or shams; one who
      attacks cherished beliefs; a radical.
      [1913 Webster]

WordNet (r) 3.0 (2006):

iconoclast
    n 1: a destroyer of images used in religious worship [syn:
         iconoclast, image breaker]
    2: someone who attacks cherished ideas or traditional
       institutions

Moby Thesaurus II by Grady Ward, 1.0:

25 Moby Thesaurus words for "iconoclast":
   annihilator, antichristian, arsonist, biblioclast, bomber, breaker,
   burner, demolisher, destroyer, dynamitard, dynamiter, earthling,
   exterminator, hun, idol breaker, idoloclast, irreligionist,
   materialist, nihilist, ruiner, syndicalist, terrorist, vandal,
   worldling, wrecker



The Devil's Dictionary (1881-1906):

ICONOCLAST, n.  A breaker of idols, the worshipers whereof are
imperfectly gratified by the performance, and most strenuously protest
that he unbuildeth but doth not reedify, that he pulleth down but
pileth not up.  For the poor things would have other idols in place of
those he thwacketh upon the mazzard and dispelleth.  But the
iconoclast saith:  "Ye shall have none at all, for ye need them not;
and if the rebuilder fooleth round hereabout, behold I will depress
the head of him and sit thereon till he squawk it."