Runoff is water from rain or melting snow that flows across the ground instead of soaking into the soil. When a big storm hits, you might notice water rushing down your driveway or pooling in low spots: that's runoff in action. The water runs off because the ground can't absorb it fast enough, or because surfaces like pavement and rooftops don't absorb water at all.

Runoff matters because it carries whatever it passes over. Water running across a farm field might pick up soil and fertilizer, carrying them into nearby streams. In cities, runoff can wash oil, trash, and other pollution from streets into storm drains that lead to rivers and lakes. Too much runoff can cause flooding when water accumulates faster than it can drain away.

The word also describes a second election held when no candidate wins enough votes in the first election. If three candidates run for student council president and no one gets more than half the votes, there might be a runoff election between the top two candidates. This helps ensure the winner has clear support from a majority of voters.