spy definition

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
2. a secret watcher; someone who secretly watches other people
• examples: my spies tell me that you had a good time last night
3. watch, observe, or inquire secretly
4. catch sight of; to perceive with the eyes
• examples: he caught sight of the king's men coming over the ridge
5. secretly collect sensitive or classified information; engage in espionage
• examples: spy for the Russians