contention vs assertion

contention and assertion both are nouns.

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

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

contention: a point asserted as part of an argument
assertion: a declaration that is made emphatically (as if no supporting evidence were necessary)

Other hypernyms of contention include asseveration, averment.

contention (noun)	assertion (noun)
the act of competing as for profit or a prize	a declaration that is made emphatically (as if no supporting evidence were necessary)
a contentious speech act; a dispute where there is strong disagreement	the act of affirming or asserting or stating something
a point asserted as part of an argument