ipse dixit vs assertion

ipse dixit and assertion both are nouns.

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

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

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

Other hypernyms of ipse dixit include asseveration, averment.

ipse dixit (noun)	assertion (noun)
an unsupported dogmatic assertion	a declaration that is made emphatically (as if no supporting evidence were necessary)
	the act of affirming or asserting or stating something