activity vs affair

activity

noun
  • Something done as an action or a movement. 

  • The lexical aspect (aktionsart) of verbs or predicates that change over time and have no natural end point. 

  • Something done for pleasure or entertainment, especially one involving movement or an excursion. 

  • The state or quality of being active; activeness. 

  • The number of radioactive decays per unit time. Unit for it: becquerel or curie 

  • The propery of substances to react with other substances 

affair

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

  • 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). 

  • 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. 

  • A material object (vaguely designated). 

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