The Collaborative International Dictionary of English v.0.48:

Radicalism \Rad"i*cal*ism\ (r[a^]d"[i^]*kal*[i^]z'm), n. [Cf. F.
   radicalisme.]
   The quality or state of being radical; specifically, the
   doctrines or principles of radicals in politics or social
   reform.
   [1913 Webster]

         Radicalism means root work; the uprooting of all
         falsehoods and abuses.                   --F. W.
                                                  Robertson.
   [1913 Webster]

WordNet (r) 3.0 (2006):

radicalism
    n 1: the political orientation of those who favor revolutionary
         change in government and society

Moby Thesaurus II by Grady Ward, 1.0:

77 Moby Thesaurus words for "radicalism":
   Fabianism, Jacobinism, New Left, Old Left, abandon, anarchism,
   anarcho-syndicalism, boundlessness, conversion,
   criminal syndicalism, egregiousness, enormousness, exaggeration,
   excess, excessiveness, exorbitance, exorbitancy, extravagance,
   extravagancy, extreme, extreme left, extreme left wing, extremes,
   extremism, extremity, fabulousness, giantism, gigantism, gluttony,
   gradualism, hyperbole, hypertrophy, immoderacy, immoderateness,
   immoderation, incontinence, inordinacy, inordinance,
   inordinateness, intemperance, intemperateness, left-wing extremism,
   meliorism, monstrousness, nihilism, nimiety, outrageousness,
   overdevelopment, overgreatness, overgrowth, overindulgence,
   overlargeness, overmuch, overmuchness, progressivism,
   radical reform, radicalization, reform, reformation, reformism,
   regeneration, revisionism, revolution, revolutionism,
   sans-culotterie, sans-culottism, syndicalism, too much,
   too-muchness, transformation, ultraconservatism, ultraism,
   unconscionableness, undueness, unreasonableness, unrestrainedness,
   utopianism



The Devil's Dictionary (1881-1906):

RADICALISM, n.  The conservatism of to-morrow injected into the
affairs of to-day.