n (military) a secret agent hired by a state to obtain information about its enemies or by a business to obtain industrial secrets from competitors
n a secret watcher; someone who secretly watches other people
v catch sight of
v watch, observe, or inquire secretly
v catch sight of; to perceive with the eyes
v secretly collect sensitive or classified information; engage in espionage
n. 间谍；密探（spy的复数形式）