A 'pen name' is the name a writer uses on the things he writes. It doesn't have to be
his real name.
The French phrase is "nom de plume" . . . "name of the pen".
An example:
In the 19th Century (the 18 hundreds), there were several great French novels
that were written by George Sand, and they became very popular.
Except that George Sand was really a lady named Armandine Aurore Lucille Dupin,
who was married and had two children.