crown prince vs tsar

crown prince

noun
  • A person designated and raised to become the next emperor. 

  • A person expected to inherit or take over an enterprise or undertaking once its current manager retires or dies. 

  • A person designated and raised to become the next king. 

tsar

noun
  • A person with great power; an autocrat. 

  • An emperor of Russia (1547 to 1917) and of some South Slavic states. 

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