What is the meaning of spying?

noun

• Keeping a secret or furtive watch.
• The act of keeping a secret watch for intelligence purposes.
  • synonyms: undercover work
• The act of detecting something; catching sight of something.
  • synonyms: detection, catching, espial, spotting

verb

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