The very basic and simplified features of imperialism are as follows:

1. The presence of monopolies which play a decisive role in economic life.

2. The merging of bank capital with industrial capital into finance capital controlled by a financial oligarchy.

3. The export of capital as distinguished from the simple export of commodities.

4. The formation of international monopolist capitalist associations (cartels) and multinational corporations.

5. The domination and exploitation of other countries by militaristic imperialist powers, now through neocolonialism.

6. The territorial division of the whole world among the biggest capitalist powers.

This is of course generalized, and each individual country's role in imperialism/anti-imperialism is going to fulfill different manifestations of this. For example, the US Empire is the hegemon, with Europe and the collective "west" vassalized post-WWII.