Definition, synonyms and related words
plural of spy
"The intelligence report detailed how several spies were operating within the capital city to gather secret information."
Third-person singular simple present indicative form of spy
"The detective noticed that a spy was watching him from across the street."
In plain English: To spy means to secretly watch someone without them knowing you are there.
"The intelligence officer will spy on our headquarters from across the border to gather secrets without being seen."