(v): catch sight of; to perceive with the eyes
(v): watch, observe, or inquire secretly
(v): keep tabs on; keep an eye on; keep under surveillance
(v): catch sight of; to perceive with the eyes
(v): to become aware of through the senses
(v): secretly collect sensitive or classified information; engage in espionage
(v): conduct an inquiry or investigation of
(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 person secretly employed in espionage for a government
(n): a secret watcher; someone who secretly watches other people
(n): a close observer; someone who looks at something (such as an exhibition of some kind)