A person who secretly plans something harmful or illegal with others; a conspirator.
From conspire + -er (one who does something). This is a straightforward English agent noun, though 'conspirator' (from Latin) is more formal.
Both 'conspirer' and 'conspirator' exist in English, but they represent different word-building traditions—'conspirer' uses English word-building rules while 'conspirator' keeps the Latinate form, yet they mean exactly the same thing.
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