introduction vs severance

introduction

noun
  • A written or oral explanation of what constitutes the basis of an issue. 

  • The act or process of introducing. 

  • A means, such as a personal letter, of presenting one person to another. 

  • An initial section of a book or article, which introduces the subject material. 

severance

noun
  • A separation. 

  • The act of severing or the state of being severed. 

  • A severance payment. 

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