runoff vs semifinal

runoff

noun
  • A second or further round of a competition, after other competitors (often all but the last two) have been eliminated. 

  • That portion of precipitation or irrigation on an area which does not infiltrate or evaporate, but instead is discharged from the area. 

  • A second or further round of an indecisive election, after other candidates (often all but the last two) have been eliminated. 

  • Dissolved chemicals, etc, included in such water. 

semifinal

noun
  • A competition that narrows a field of contestants (semifinalists) to a set of finalists, for a subsequent final. 

  • A playoff in the round with only four players or teams left, the stage before the final. 

How often have the words runoff and semifinal occurred in a corpus of books? (source: Google Ngram Viewer )