spy · noun C /spɑɪ/
spy
B1 someone who secretly tries to discover information about a person, country, etc.
Dictionary example:
His father was a British spy.
Learner example:
It's a spy story. (Preliminary English Test; B1; Italian)
spy · verb /spɑɪ/
spy
C1 I to secretly try to discover information about a person, country, etc.
Dictionary example:
He was accused of spying.
Learner example:
Also, some countries have fears about foreigners coming from underdeveloped countries and may consider some of them as a source of troubles including crimes, spying and maybe terrorism. (International English Language Testing System; C1; Arabic)
spy on sb
C2 to secretly watch someone
Dictionary example:
He spied on her through the keyhole.
Learner example:
Edmund starts spying on Charles to find out which his weaknesses are. (Certificate of Proficiency in English; C2; Spanish)
