affair vs undertaking

affair

noun
  • Something which is done or is to be done; business of any kind, commercial, professional, or public. 

  • An action or engagement not of sufficient magnitude to be called a battle. 

  • An adulterous relationship (from affaire de cœur, affair of the heart). 

  • A person with whom someone has an adulterous relationship. 

  • A romantic relationship with someone who is not one's regular partner (boyfriend, girlfriend). 

  • A party or social gathering, especially of a formal nature. 

  • Any proceeding or action which it is wished to refer to or characterize vaguely. 

  • A material object (vaguely designated). 

undertaking

noun
  • That which is undertaken; any business, work, or project which a person engages in, or attempts to perform; an enterprise. 

  • A promise or pledge; a guarantee. 

  • The business of an undertaker, or the management of funerals. 

  • The act of one who undertakes (in either sense). 

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