election vs runoff

election and runoff both are nouns.

Word	Noun	Adjective	Verb	Adverb
election	Yes	No	No	No
runoff	Yes	No	No	No

As nouns, runoff is a hyponym of election; that is, runoff is a word with a more specific, narrower meaning than election:

election: a vote to select the winner of a position or political office
runoff: a final election to resolve an earlier election that did not produce a winner

Other hyponyms of election include reelection, general election, primary, primary election, by-election, bye-election.

election (noun)	runoff (noun)
the act of selecting someone or something; the exercise of deliberate choice	a final election to resolve an earlier election that did not produce a winner
a vote to select the winner of a position or political office	the occurrence of surplus liquid (as water) exceeding the limit or capacity
the predestination of some individuals as objects of divine mercy (especially as conceived by Calvinists)
the status or fact of being elected