Nom de plume
Nom de plume is the French equivalent of pen-name. Literally, the French is 'name of [or by] pen'. Both denote a pseudonym adopted for the purposes of writing. Some writers choose to publish under an alternative name, to avoid publicity, to keep their authorship of a writing secret from their social circle, or to avoid censorship or persecution by the authorities.
Some academics prefer to use the French phrase, but this is falling into disuse. The English pen name is preferable, to our taste.