crown princess vs empress

crown princess

noun
  • The heiress apparent to the throne in a royal or imperial monarchy. 

  • The wife of a crown prince. 

empress

noun
  • The female monarch (ruler) of an empire. 

  • The third trump or major arcana card of most tarot decks. 

  • A female chimpanzee. 

  • A deciduous tree, Paulownia tomentosa 

  • The wife or widow of an emperor or equated ruler. 

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