Imperialism is the domination of a land and its people by an external power. It is the antithesis of self-determination. It is closely tied to colonialism, although scholars often distinguish between imperialism, in which the ruling power remains external to the dominated people, and colonialism, in which the ruling power sends its people to settle and rule from the conquered colony. Others treat imperialism as the ideology of expansion through use of force and see colonialism as one way imperialism looks in practice.
History is, unfortunately, full of examples of imperialism. The British empire engaged in major imperialist endeavors in Ireland, India, the lands that are now known as the United States and Canada, Australia, New Zealand, and throughout the Caribbean. We can distinguish between cases such as India, in which indigenous people were subjugated by a small, ruling British minority (which was eventually repelled), and cases such as the United States, in which the indigenous people were exterminated to the extent that they became a small minority in their lands.
We should be careful to distinguish between European settlers seeking independence from the states that founded their countries (as happened in the revolutionary war) and the struggle between indigenous people and imperial powers seeking to dominate them. While conditions for poor British settlers in the colonies that are now the United States may have included mistreatment and poverty, it would be a mistake to call this an imperial relationship, as a crucial part of imperialism is that one power is dominating another people, their land, and their resources.
Centering the power struggle between European settlers and Britain obscures the fact that both were committed to the extermination and/or subjugation of the indigenous people of North America in order to gain access to their land and extract its resources. Such instances can be understood as examples of settler colonialism, in which a core function of colonies is to provide a place for members of the ruling colonial power to settler. Settler colonies often experience conflict between settlers and the colonial power, with both sides competing to wield imperialist power over the indigenous people of the land where the colonies exist.
While the specifics of imperialism vary the history of world powers is the history of imperialism: from the Roman Empire to the Ottoman Empire, European colonial powers, Japan, the United States, and the USSR. States wielding their power to subjugate others has been nearly omnipresent over the course of recorded history.