A runoff is an additional election that resolves an inconclusive one. If there’s a tie for class president, the candidates will need a runoff election to see who gets the job. A runoff is also an overflow of water.
The physical type of runoff (also run-off) involves water or other liquids running off of something by overflowing and possibly flooding an area. If it rains and rains, the runoff from the soil could flow into a nearby pond. On the other hand, a runoff election means an initial vote did not result in a winner because it was tied or too close to call. A runoff election settles the matter, providing a final decision.