the.com/runoff

means A second, deciding round of voting held when no candidate won the majority required in the first round — or, in its older sense, the water that spills off land rather than soaking in.

from The 'run' here is the plain Old English verb meaning to flow, and 'runoff' first described water or melted snow running off the surface of land rather than being absorbed — a term that settled into English usage with the general literal sense of 'that which runs off.' The electoral meaning is a later American extension of the same image: votes that didn't get absorbed into a majority the first time around simply run off into a second contest.