spy

nouns

(military) a secret agent hired by a state to obtain information about its enemies or by a business to obtain industrial secrets from competitors
a secret watcher; someone who secretly watches other people
"my spies tell me that you had a good time last night"
spy
is a type of (narrower)
a close observer; someone who looks at something (such as an exhibition of some kind)
"the spectators applauded the performance"; "television viewers"; "sky watchers discovered a new star"

verbs

catch sight of
is derived from
watch, observe, or inquire secretly
is derived from
catch sight of; to perceive with the eyes
"he caught sight of the king's men coming over the ridge"
secretly collect sensitive or classified information; engage in espionage
"spy for the Russians"
spy
is derived from
is a type of (narrower)
conduct an inquiry or investigation of
"The district attorney's office investigated reports of possible irregularities"; "inquire into the disappearance of the rich old lady"