Etymology
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imperialism (n.)

1826, "advocacy of empire, devotion to imperial interests," originally in a Napoleonic context, also of Rome and of British foreign policy; from imperial + -ism. At times in British usage (and briefly in U.S.) with a neutral or positive sense relating to national interests or the spread of the benefits of Western civilization, but from the beginning usually more or less a term of reproach. General sense of "one country's rule over another," first recorded 1878. 

It is the old story of 1798, when French republicanism sick of its own folly and misdeeds, became metamorphosed into imperialism, and consoled itself for its incapacity to found domestic freedom by putting an iron yoke upon Europe, and covering it with blood and battle-fields. [Francis Lloyd, St. James's Magazine, January 1842]

The word sharpened its pejorative after 1900, with U.S. revulsion to Philippine insurrection and the publication of J.A. Hobson's book on imperialism, through which the term was taken up in the 1910s by communist writers.

Imperialism is a depraved choice of national life, imposed by self-seeking interests which appeal to the lusts of quantitative acquisitiveness and of forceful domination surviving in a nation from early centuries of animal struggle for existence. Its adoption as a policy implies a deliberate renunciation of that cultivation of the higher inner qualities which for a nation as for an individual constitutes the ascendency of reason over brute impulse. It is the besetting sin of all successful States, and its penalty is unalterable in the order of nature. [J.A. Hobson, "Imperialism: A Study," London, 1902]

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Definitions of imperialism

imperialism (n.)
a policy of extending your rule over foreign countries;
imperialism (n.)
a political orientation that advocates imperial interests;
imperialism (n.)
any instance of aggressive extension of authority;
From wordnet.princeton.edu