1a further competition, election, race, etc., after a tie or inconclusive result:he won only 49 per cent of the vote, so a run-off will be held
2 [mass noun] the draining away of water (or substances carried in it) from the surface of an area of land, a building or structure, etc.:the ratio of run-off to rainfallfertilizer run-off from the intensively farmed areas in the river basin
the water or other material that drains freely off the surface of something:when it rains the run-off cascades down to the valley floor [count noun]:water polluted by run-offs from farmland
3NZ a separate area of land where young animals are kept.
run-off in other Oxford dictionaries
Definition of run-off in the US English dictionary