WordWeb Online

1. Keeping a secret or furtive watch
    
2. The act of keeping a secret watch for intelligence purposes
   - undercover work
    
3. The act of detecting something; catching sight of something
   - detection, catching, espial, spotting

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

Derived forms: spyings

Type of: comprehend, discovery, enquire [Brit, Cdn], find, inquire, intelligence, intelligence activity, intelligence operation, investigate, monitor, perceive, supervise, uncovering, vigil, watch, watchdog

Encyclopedia: Spying