acronym vs initialism

acronym

noun
  • An abbreviation formed by the beginning letters or syllables of other words (as "Benelux"). 

  • An abbreviation formed by the initial letters of other words, sometimes exclusively such abbreviations when pronounced as a word (as "laser") rather than as individual letters (initialisms such as "TNT"). 

verb
  • To form into an acronym. 

initialism

noun
  • A term formed from the initial letters of several words or parts of words, which is itself pronounced letter by letter. 

  • The process of forming words or terms using initial letters of other words. 

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