spy in WordNet English dictionary

1. catch sight of
   descry  spot  espy  spy 
   
2. catch sight of; to perceive with the eyes
   "he caught sight of the king's men coming over the ridge"
   spy  sight 
   
3. watch, observe, or inquire secretly
   spy  stag  snoop  sleuth 
   
4. secretly collect sensitive or classified information; engage in espionage
   "spy for the Russians"
   spy 
   

1. (military) a secret agent hired by a state to obtain information about its enemies or by a business to obtain industrial secrets from competitors
   spy  undercover agent 
   
2. a secret watcher; someone who secretly watches other people
   "my spies tell me that you had a good time last night"
   spy