monarch vs queen mother

monarch

noun
  • The ruler of an absolute monarchy or the head of state of a constitutional monarchy. 

  • The monarch butterfly, Danaus plexippus, and others of genus Danaus, found primarily in North America, so called because of the designs on its wings. 

  • A stag which has sixteen or more points or tines on its antlers. 

  • The chief or best thing of its kind. 

  • A police officer. 

  • Any bird of the family Monarchidae. 

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 monarch and queen mother occurred in a corpus of books? (source: Google Ngram Viewer )