initialism vs initials

initialism

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

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

initials

noun
  • An abbreviation of a person's name, constructed by taking the initial letters of first, last, and sometimes middle name(s). See monogram 

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