• WordNet 3.6
    • n separationist an advocate of secession or separation from a larger group (such as an established church or a national union)
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Century Dictionary and Cyclopedia
    • n separationist One who advocates or favors separation, in some special sense.
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In literature:

And he reflected that Decoud's separationist plan had not gone to the bottom with the lost silver.
"Nostromo: A Tale of the Seaboard" by Joseph Conrad
You yourself are a Separationist, are you not?...
"Romance" by Joseph Conrad and F.M. Hueffer