the.com/pen name

means A false name a writer puts on their work instead of their own.

from A half-translated phrase — 'pen' is plain English for the writer's tool, but 'nom de plume' is actually a 19th-century English invention dressed up in French, not a genuine French expression. The real French phrase is 'nom de guerre' (war name), originally used by soldiers and actors adopting battle aliases, which English speakers borrowed and then, oddly, re-fitted with 'plume' (feather/pen) to make it sound more literary and more French than it actually was.