nom de plume vs anonym

nom de plume and anonym both are nouns.

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

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

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

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

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