nom de plume vs pseudonym

nom de plume and pseudonym both are nouns.

Word	Noun	Adjective	Verb	Adverb
nom de plume	Yes	No	No	No
pseudonym	Yes	No	No	No

As nouns, pseudonym is a hypernym of nom de plume; that is, pseudonym is a word with a broader meaning than nom de plume:

nom de plume: an author's pseudonym
pseudonym: a fictitious name used when the person performs a particular social role

Other hypernyms of nom de plume include anonym, nom de guerre.

nom de plume (noun)	pseudonym (noun)
an author's pseudonym	a fictitious name used when the person performs a particular social role