baroness vs queen mother

baroness

noun
  • A woman holding a baronial title in her own right; a female ruler of a barony. 

  • The wife of a baron. 

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