Source : Webster's Revised Unabridged Dictionary (1913)

Imperialism \Im*pe"ri*al*ism\, n.
   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 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.

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

         Roman imperialism had divided the world. --C. H.
                                                  Pearson.

Source : WordNet®

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