a person employed by a government to obtain secret information or intelligence about another, usually hostile, country, esp. with reference to military or naval affairs.

a person who keeps close and secret watch on the actions and words of another or others.

a person who seeks to obtain confidential information about the activities, plans, methods, etc., of an organization or person, esp. one who is employed for this purpose by a competitor

an industrial spy.

the act of spying.

to observe secretively or furtively with hostile intent (often fol. by on or upon ).

to act as a spy; engage in espionage.

to be on the lookout; keep watch.

to search for or examine something closely or carefully.

to catch sight of suddenly; espy; descry

to spy a rare bird overhead.

to discover or find out by observation or scrutiny (often fol. by out ).

to observe (a person, place, enemy, etc.) secretively or furtively with hostile intent.

to inspect or examine or to search or look for closely or carefully.

noun

agent , detective , double agent , emissary , espionage agent , foreign agent , informer , inside agent , intelligencer , investigator , lookout , mole * , observer , operative , patrol , picket , plant * , scout , secret agent , secret service , sleeper , sleuth , snoop , spook , spotter , undercover agent , watcher

verb

case , catch sight of , discover , examine , eyeball , fish out , get a load of , glimpse , keep under surveillance , look for , meddle , notice , observe , peep , pry , recon , reconnoiter , scout , scrutinize , search , set eyes on , shadow , sleuth , snoop , spot , stag , stake out , tail , take in , take note , trail , view , watch , eavesdrop , descry , detect , discern , espy , agent , behold , informer , inspect , mole , saboteur , secret agent , see , sleeper , sneak , snooper , stoolie