Spy.
Synonyms:
detective, emissary, scout.
The scout and the spy are both employed to obtain information of the numbers, movements, etc., of an enemy. The scout lurks on the outskirts of the hostile army with such concealment as the case admits of, but without disguise; a spy enters in disguise within the enemy's lines. A scout, if captured, has the rights of a prisoner of war; a spy is held to have forfeited all rights, and is liable, in case of capture, to capital punishment. An emissary is rather political than military; sent rather to secretly influence opponents than to bring information concerning them; so far as he does the latter, he is not only an emissary, but a spy.