According to the Oxford English Dictionary, (1) "The practice or manner of things colonial" and often serves as a synonym for "provincial;" (2) "The colonial system or principle."
Now the term frequently used in the derogatory sense of an alleged "policy of exploitation of backward or weak peoples by a large power."
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According to the Stanford Encyclopedia of Philosophy, Colonialism is a practice of domination, which involves the subjugation of one people to another. One of the difficulties in defining colonialism is that it is difficult to distinguish it from imperialism.
Frequently the two concepts are treated as synonyms. Like colonialism, imperialism also involves political and economic control over a dependent territory. Turning to the etymology of the two terms, however, provides some suggestion about how they differ.
The term colony comes from the Latin word colonus, meaning farmer. This root reminds us that the practice of colonialism usually involved the transfer of population to a new territory, where the new arrivals lived as permanent settlers while maintaining political allegiance to their country of origin.
Imperialism, on the other hand, comes from the Latin term imperium, meaning to command. Thus, the term imperialism draws attention to the way that one country exercises power over another, whether through settlement, sovereignty, or indirect mechanisms of control.
A detailed review of the concept is available at: [| Stanford Encyclopedia - Colonialism