spy

(noun) (military) a secret agent hired by a state to obtain information about its enemies or by a business to obtain industrial secrets from competitors. Synonyms: undercover agent.

(noun) a secret watcher; someone who secretly watches other people. Examples: "My spies tell me that you had a good time last night."

(verb) catch sight of. Synonyms: descry, spot, espy.

(verb) watch, observe, or inquire secretly. Synonyms: stag, snoop, sleuth.

(verb) catch sight of; to perceive with the eyes. Synonyms: sight. Examples: "He caught sight of the king's men coming over the ridge."

(verb) secretly collect sensitive or classified information; engage in espionage. Examples: "Spy for the russians."