The epoch of imperialism opens when the expansion of colonialism has covered the globe and no new colonies can be acquired by the great powers except by taking them from each other,
and the concentration of capital has grown to a point where finance capital becomes dominant over industrial capital.(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 monopoly 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.
http://en.wikipedia.org/wiki/Imperialism,_the_Highest_Stage_of_Capitalism