A person who advocates for, supports, or practices colonization; someone who believes in the expansion of colonial territories.
From 'colonization' + '-ist' (suffix forming nouns for people with specific ideologies). American spelling; British equivalent is 'colonisationist.' Emerged in the 19th century.
Some colonizationists were quite philosophical about their work—they genuinely believed they were 'civilising' the world, which is a powerful reminder that harmful ideologies don't usually see themselves as harmful; they package themselves as progress.
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