"A person who secretly collects and reports information about the activities of another country or organization." (CD)
See also: https://dictionary.cambridge.org/us/dictionary/english/spy
The graph displays the other subjects mentioned on the same pages as the subject "Spies". If the same subject occurs on a page with "Spies" more than once, it appears closer to "Spies" on the graph, and is colored in a darker shade. The closer a subject is to the center, the more "related" the subjects are.