Lenin gives five basic features of imperialism:
(1) the concentration of production and capital has developed to such a high stage that it has created monopolies which play a decisive role in economic life; (2) the merging of bank capital with industrial capital, and the creation, on the basis of this “finance capital”, of a financial oligarchy; (3) the export of capital as distinguished from the export of commodities acquires exceptional importance; (4) the formation of international monopolist capitalist associations which share the world among themselves, and (5) the territorial division of the whole world among the biggest capitalist powers is completed. Imperialism is capitalism at that stage of development at which the dominance of monopolies and finance capital is established; in which the export of capital has acquired pronounced importance; in which the division of the world among the international trusts has begun, in which the division of all territories of the globe among the biggest capitalist powers has been completed. (Imperialism, the Highest Stage of Capitalism)
There are also four fundamental contradictions of imperialism. Stalin, in The Foundations of Leninism, went on to enumerate three of them: (1) the contradiction between labor and capital; (2) the contradiction among the various financial groups and imperialist Powers in their struggle for sources of raw materials; and (3) the contradiction between the handful of ruling, “civilised” nations and the hundreds of millions of the colonial and dependent peoples of the world. There is a fourth in the contemporary world as Mao Zedong and the Chinese Communists have pointed out: (4) the contradiction between imperialism and the socialist countries. Of these four contradictions, the principal contradiction in the current period is that between the oppressed nations and imperialism. These four contradictions can only be resolved by social revolution.
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For more info on these questions, two sections of the M-L Study Guide should prove particularly helpful: first, the section on the political economy of capitalism and imperialism, and second, the section on proletarian internationalism and the national-colonial question.