affirmation vs assertion

affirmation and assertion both are nouns.

Word	Noun	Adjective	Verb	Adverb
affirmation	Yes	No	No	No
assertion	Yes	No	No	No

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

affirmation: a statement asserting the existence or the truth of something
assertion: a declaration that is made emphatically (as if no supporting evidence were necessary)

Other hypernyms of affirmation include asseveration, averment.

As nouns, affirmation and assertion are synonyms defined as:

affirmation and assertion: the act of affirming or asserting or stating something

Other synonyms of affirmation include statement.

affirmation (noun)	assertion (noun)
a judgment by a higher court that the judgment of a lower court was correct and should stand	a declaration that is made emphatically (as if no supporting evidence were necessary)
a statement asserting the existence or the truth of something	the act of affirming or asserting or stating something
(religion) a solemn declaration that serves the same purpose as an oath (if an oath is objectionable to the person on religious or ethical grounds)
the act of affirming or asserting or stating something