It's election season in Brazil. How to use "runoff".

image from here

Last Monday it was the first round of Brazil´s presidential election. 

The current president Dilma Rousseff didn't get the majority necessary for a reelection and now she is facing Aécio Neves in runoff (a second election or competition that is organized when the first one does not have a winner). 

Brazil presidential vote headed for a runoff. CNN

Brazil Presidential Election Results In Runoff. Reuters

For more election vocabulary (explanation in  Portuguese), 

click on here!