ballot vs referendum

ballot

noun
  • The process of voting, especially in secret; a round of voting. 

  • The total of all the votes cast in an election. 

  • A list of candidates running for office; a ticket. 

  • Originally, a small ball placed in a container to cast a vote; now, by extension, a piece of paper or card used for this purpose, or some other means used to signify a vote. 

verb
  • To draw lots. 

  • To vote or decide by ballot. 

referendum

noun
  • A direct popular vote on a proposed law or constitutional amendment. The adposition on is usually used before the related subject of the vote. 

  • An action, choice, etc., which is perceived as passing judgment on another matter. 

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