runoff vs overspill

runoff and overspill both are nouns.

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

As nouns, runoff and overspill are synonyms defined as:

runoff and overspill: the occurrence of surplus liquid (as water) exceeding the limit or capacity

Other synonyms of runoff include overflow.

runoff (noun)	overspill (noun)
a final election to resolve an earlier election that did not produce a winner	the occurrence of surplus liquid (as water) exceeding the limit or capacity
the occurrence of surplus liquid (as water) exceeding the limit or capacity	the relocation of people from overcrowded cities; they are accommodated in new houses or apartments in smaller towns