spy
/ˈspī/
verb
to watch secretly usually for hostile purposes
to catch sight of : see
to search or look for intensively —usually used with out
noun
one that spies:
one who keeps secret watch on a person or thing to obtain information
a person employed by one nation to secretly convey classified information of strategic importance to another nation; also : a person who conveys the trade secrets of one company to another
phrasal verb
to watch (someone) secretly
He spies on his neighbors.