Real Dictionary

spy
A spy satellite or spy plane obtains secret information about another country by taking photographs from the sky.
countable noun: A spy is a person whose job is to find out secret information about another country or organization.
verb: If you spy on someone, you watch them secretly.
verb: Someone who spies for a country or organization tries to find out secret information about another country or organization.

spy in American English
a person who keeps close and secret watch on another or others
of or about spies or espionage
to watch or observe closely and secretly, usually with unfriendly purpose
to watch or observe closely and secretly; specif., to act as a spy
intransitive verb: to observe secretively or furtively with hostile intent
noun: a person employed by a government to obtain secret information or intelligence about another, usually hostile, country, esp. with reference to military or naval affairs
transitive verb: to catch sight of suddenly; espy; descry