The Collaborative International Dictionary of English v.0.48:

Colonialism \Co*lo"ni*al*ism\, n.
   1. The state or quality of, or the relationship involved in,
      being colonial.

            The last tie of colonialism which bound us to the
            mother country is broken.             --Brander
                                                  Matthews.
      [Webster 1913 Suppl.]

   2. A custom, idea, feature of government, or the like,
      characteristic of a colony.
      [Webster 1913 Suppl.]

   3. The colonial system of political government or extension
      of territory, by which one nation exerts political control
      over another nation, territory, or people, maintaining the
      colony in a state of dependence, its inhabitants not
      having the same full rights as those of the colonial
      power. The controlling power is typically extended thus by
      military force or the threat of force.
      [Webster 1913 Suppl. +PJC]

   4. the political or ideological system of beliefs advocating
      or justifying colonial control of one nation over another
      nation, territory, or people.
      [PJC]

WordNet (r) 3.0 (2006):

colonialism
    n 1: exploitation by a stronger country of weaker one; the use
         of the weaker country's resources to strengthen and enrich
         the stronger country

Moby Thesaurus II by Grady Ward, 1.0:

96 Moby Thesaurus words for "colonialism":
   Eisenhower Doctrine, Monroe Doctrine, Nixon Doctrine,
   Truman Doctrine, absolute monarchy, appeasement, aristocracy,
   autarchy, autocracy, autonomy, balance of power, brinkmanship,
   coalition government, coexistence, commonwealth, compromise,
   constitutional government, constitutional monarchy, containment,
   democracy, detente, deterrence, dictatorship, diplomacy,
   diplomatic, diplomatics, dollar diplomacy, dollar imperialism,
   dominion rule, duarchy, duumvirate, dyarchy, expansionism,
   federal government, federation, feudal system, foreign affairs,
   foreign policy, garrison state, gerontocracy, good-neighbor policy,
   heteronomy, hierarchy, hierocracy, home rule, imperialism,
   internationalism, isolationism, limited monarchy, manifest destiny,
   martial law, meritocracy, militarism, military government,
   mob rule, mobocracy, monarchy, nationalism, neocolonialism,
   neutralism, nonresistance, ochlocracy, oligarchy, open door,
   open-door policy, pantisocracy, patriarchate, patriarchy,
   peace offensive, peaceful coexistence, police state, preparedness,
   pure democracy, regency, representative democracy,
   representative government, republic, self-determination,
   self-government, shirt-sleeve diplomacy, shuttle diplomacy,
   social democracy, spheres of influence, stratocracy, technocracy,
   the big stick, thearchy, theocracy, totalitarian government,
   totalitarian regime, tough policy, triarchy, triumvirate, tyranny,
   welfare state, world politics