Etymology
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runoff (n.)

also run-off, "precipitation water drained by streams and rivers," 1887, from the verbal phrase; see run (v.) + off (adv.). The meaning "deciding race after a tie" is by 1873; the extended sense in reference to an election between the two who got the most votes in the previous, undecided election is attested by 1910, American English.

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Definitions of runoff

runoff (n.)
the occurrence of surplus liquid (as water) exceeding the limit or capacity;
Synonyms: overflow / overspill
runoff (n.)
a final election to resolve an earlier election that did not produce a winner;
From wordnet.princeton.edu