ipse dixit vs asseveration

ipse dixit and asseveration both are nouns.

Word	Noun	Adjective	Verb	Adverb
ipse dixit	Yes	No	No	No
asseveration	Yes	No	No	No

As nouns, asseveration is a hypernym of ipse dixit; that is, asseveration is a word with a broader meaning than ipse dixit:

ipse dixit: an unsupported dogmatic assertion
asseveration: a declaration that is made emphatically (as if no supporting evidence were necessary)

Other hypernyms of ipse dixit include assertion, averment.

ipse dixit (noun)	asseveration (noun)
an unsupported dogmatic assertion	a declaration that is made emphatically (as if no supporting evidence were necessary)