Imperialism is when a powerful country seeks to control or dominate [have power or authority over] other countries or regions. This can be achieved through political, economic, or military means. For example, European countries colonizing parts of Africa and Asia in the 19th and 20th centuries to exploit their resources and establish trade networks was one form of Imperialism.
Very Simple Definition:
Imperialism is when one country takes control of other countries or regions. An example of Imperialism is Napoleon trying to conquer all of Europe.