NOUN (3)

1. a policy of extending your rule over foreign countries;

2. a political orientation that advocates imperial interests;

3. any instance of aggressive extension of authority;

The Collaborative International Dictionary of English v.0.48:

Imperialism \Im*pe"ri*al*ism\, n.
   1. The power or character of an emperor; imperial authority;
      the spirit of empire.
      [1913 Webster]

            Roman imperialism had divided the world. --C. H.
                                                  Pearson.
      [1913 Webster]

   2. The policy, practice, or advocacy of seeking, or
      acquiescing in, the extension of the control, dominion, or
      empire of a nation, as by the acquirement of new, esp.
      distant, territory or dependencies, or by the closer union
      of parts more or less independent of each other for
      operations of war, copyright, internal commerce, etc. The
      practise of building or extending an empire.
      [Webster 1913 Suppl. +PJC]

            The tide of English opinion began to turn about
            1870, and since then it has run with increasing
            force in the direction of what is called
            imperialism.                          --James Bryce.
      [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

imperialism
    n 1: a policy of extending your rule over foreign countries
    2: a political orientation that advocates imperial interests
    3: any instance of aggressive extension of authority

Moby Thesaurus II by Grady Ward, 1.0:

83 Moby Thesaurus words for "imperialism":
   Eisenhower Doctrine, Monroe Doctrine, Nazism, Nixon Doctrine,
   Truman Doctrine, appeasement, balance of power, brinkmanship,
   caliphate, centralism, coexistence, collectivism, colonialism,
   communism, compromise, constitutionalism, containment, czardom,
   democratism, detente, deterrence, diplomacy, diplomatic,
   diplomatics, dollar diplomacy, dollar imperialism, emperorship,
   empery, empire, expansionism, fascism, federalism, feudalism,
   feudality, foreign affairs, foreign policy, good-neighbor policy,
   governmentalism, internationalism, isolationism, kaiserdom,
   kaisership, kinghood, kingship, majesty, manifest destiny,
   militarism, monarchism, national socialism, nationalism,
   neocolonialism, neofascism, neutralism, nonresistance, open door,
   open-door policy, parliamentarianism, parliamentarism,
   peace offensive, peaceful coexistence, pluralism,
   political principles, preparedness, queenhood, queenship, rajaship,
   regnancy, republicanism, royalism, royalty, shirt-sleeve diplomacy,
   shuttle diplomacy, socialism, sovereignty, spheres of influence,
   statism, sultanate, sultanship, the big stick, the crown,
   the throne, tough policy, world politics