infiltrator

Estimated CEFR level: C2 — Proficiency

Estimated from word frequency; not an official CEFR classification.

Definition

noun someone who takes up a position surreptitiously for the purpose of espionage
noun an intruder (as troops) with hostile intent

From infiltrate + -or.

To guarantee their loyalty to the cause, whatever that cause might be, and to weed out potential external infiltrators, each member had to have killed at least one member of their immediate family. — Terminal Compromise, Winn Schwartau

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