initialism vs short form

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. 

short form

noun
  • A word with the same meaning as another formed by removing one or more of the syllables of the longer word, and considered a word in its own right rather than an abbreviation. 

  • Particularly in Slavic languages, a shortened form of the adjective, used predicatively. 

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