A Contaminant (異物 ibutsu "contaminant") is any existence that doesn't belong to the original world a Phantom Field was constructed from (or based off of), and can include existences that were "pulled" into the Phantom Field after its creation, not just those that Dive in from outside.[1]
Phantom Fields (or the Power of Order that governs them and ensures the world's continued existence[2]) reject Contaminants and will attempt to purge them, (and in Dark War) usually by sending packs of seithr-infested creatures and monsters generated from seithr after them.
Compare with Strangers, who are people who purposefully infiltrate a Phantom Field or world from outside.
Members
Notably, Naoto was a Contaminant in Central Fiction.