concubine vs lover

concubine

noun
  • A sexual partner, especially a woman, to whom one is not or cannot be married. 

  • A slave-girl or woman, kept for instance in a harem, who is held for sexual service. 

  • A woman who lives with a man, but who is not a wife. 

lover

noun
  • A sexual partner, especially one with whom someone is having an affair. 

  • A person who loves something. 

  • An informal term of address for any friend. 

  • One who loves and cares for another person in a romantic way; a sweetheart, love, soulmate, boyfriend, girlfriend, or spouse. 

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