Longman Dictionary of Contemporary English
n. (context business espionage military English) A name used to clandestinely identify somebody or something, either for military or espionage purposes, or for commercial confidentiality.
A ' code name' or cryptonym is a word or name used, sometimes clandestinely, to refer to another name, word, project or person. Names are often used for military purposes, or in espionage. They may also be used in industrial counter- industrial espionage to protect secret projects and the like from business rivals, or to give names to projects whose marketing name has not yet been determined. Another reason for the use of names and phrases in the military is that they transmit with a lower level of cumulative errors over a walkie-talkie or radio link than actual names.
Usage examples of "code name".
Captain Prince Lysander, code name Kicker Six, who would be leading the scout element.
Her staff found her code name ironic for several reasons, but mostly because she was not in the least charming.
And now she knew for sure the code name Starscream was a password into secret files inside the Infirmary.