What is meant by the term Imperialism?

Imperialism is the policy by which a powerful nation establishes its control over another country — by political pressure, economic penetration, infiltration or outright war — and holds it as a COLONY, exploiting its raw materials, its markets and its people for the benefit of the mother country.

Two marks, so the definition needs two elements: the means (pressure, penetration or war) and the purpose (holding the territory as a colony for economic exploitation). The link to the question worth adding: by 1914 the industrial powers were competing fiercely for colonies in Africa and Asia, and Germany, a late entrant, resented the share held by Britain and France. That rivalry is why imperialism appears among the causes of the First World War.