empress vs queen mother

empress

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

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

  • The female monarch (ruler) of an empire. 

  • A female chimpanzee. 

  • A deciduous tree, Paulownia tomentosa 

queen mother

noun
  • A widowed queen consort (a queen dowager) whose son or daughter from that marriage is the reigning monarch. 

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