These words all denote a person who secretly gathers information about persons or about countries other than his own, usually for destructive purposes. Spy is the most general term. In a narrow sense, a spy is one engaged in espionage ?that is, his is sent secretly into another country to obtain military or political information wanted by his own country. If captured, especially in time of war, a spy is subject to death by firing squad. In a wider sense the word is derogatory and is applied to one who uses underhand and furtive means to observe the activities of others, usually for personal gain: The director has spies in every department.

Agent and secret agent are now preferred to spy when signifying a person employed by a government to engage in espionage. An agent or secret agent enters a foreign country under a disguise and usually resides there for a time. His work is to learn the military secrets and other facts about that country which will be of use to his own government. An agent may also be sent within the borders of a belligerent power to commit acts of sabotage that will weaken the defences of the enemy. Agent or secret agent may be applied also to investigators within a government who probe into treasonable activities, counterfeiting and other infringements of the law: a Security Organization agent .

A counterspy is an agent who spies on the secret agents of the enemy, often within his own country, in order to thwart their activities and destroy their efficiency.

The double agent , or "double spy ," is employed simultaneously by two opposing countries and engages in espionage against both of them while pretending to be working for only one. A double agent may actually be loyal to one country or he may practise deception towards both sides. Double agents often make effective counterspies .

Agent provocateur is a French term designating a spy who is planted in a trade union, political party or other organization in which there are conflicting loyalties. His method is to gain the trust of the members of the group and to stir them to actions or declarations that will incur punishment. Agent provocateur carries also a suggestion of opprobrium, since, unlike the espionage agent, the agent provocateur may be acting against and betraying his own kind.