affair vs relationship

affair

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

  • 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. 

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

  • 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). 

relationship

noun
  • A romantic or sexual involvement. 

  • The level or degree of affinity between keys, chords and tones. 

  • The links between the x-values and y-values of ordered pairs of numbers especially coordinates. 

  • Connection or association; the condition of being related. 

  • Kinship; being related by blood or marriage. 

  • A way in which two or more people behave and are involved with each other 

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