spy

Noun

• a secret watcher; someone who secretly watches other people
  • my spies tell me that you had a good time last night
    

• (military) a secret agent hired by a state to obtain information about its enemies or by a business to obtain industrial secrets from competitors

Verb

• secretly collect sensitive or classified information; engage in espionage
  • spy for the Russians
    

• catch sight of; to perceive with the eyes
  • he caught sight of the king's men coming over the ridge
    

• watch, observe, or inquire secretly

• catch sight of

Related words

• Synonyms: 
  undercover agent , sight , stag , snoop , sleuth , descry , spot , espy