One who confederates or enters into a confederation; an archaic term for a confederator.
From confederate + the agent noun suffix -er (one who performs an action). This form is largely obsolete, replaced by 'confederator.'
The existence of both 'confederater' and 'confederator' shows English speakers debating which suffix felt more natural for creating agent nouns—eventually '-or' (from Latin) won out over '-er' in more formal contexts.
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