Imperialism has two meanings, one describing an action and the other describing an attitude. Most commonly it is understood in relation to Empire building, as the expansion of a nation's authority by territorial conquest establishing economic and political powers in other territories or nations, and when such encompasses non-contiguous "colonies" or "protectorates" then the term also subsumes Colonialism. In that sense, most European seafaring powers were at one time or another Colonialistic and therefore Imperialistic, regardless of their exploitation or benevolence toward their colonial possessions and people.
In its second meaning the term describes the imperialistic attitude of superiority, subordination and dominion over foreign people— a chauvinism and comportment relegating foreign people to a lesser social and or political status. To clarify the distinction, the French colonies in North America treated the native races with great diplomacy, whereas the British and British colonies early on began treating native Americans chauvinistically, as savages and lesser creatures or fit only to be exterminated. In contrast, the abhorrent record of the Spanish made the British behavior look almost tolerant and good, and the Japanese Empire's treatment of conquered people was at best brutal.
Imperialism is often autocratic, e.g. in early 20th century Japan, and sometimes monolithic  in character. While the term imperialism often refers to a contigous political or geographical domain such as the Ottoman Empire the Russian Empire, or the British Empire, etcetera, the term can equally be applied to domains of knowledge, beliefs, values and expertise, such as the empires of Christianity (see Christendom) or Islam (see Caliphate).