[ih-nish-uh-liz-uh m]
  1. a set of initials representing a name, organization, or the like, with each letter pronounced separately, as FBI for Federal Bureau of Investigation.
  2. a name or term formed from the initial letters of a group of words and pronounced as a separate word, as NATO for North Atlantic Treaty Organization; an acronym.
  3. the practice of using initials or forming words from initials.

Origin of initialism

First recorded in 1895–1900; initial + -ism
Can be confusedabbreviation acronym initialism

Grammar note Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

Word Origin and History for initialism

word formed from the first letters of other words or a phrase, 1957, from initial (n.) + -ism. The distinction from acronym is not universally agreed-upon; in general, words such as NATO, where the letters form a word, are regarded as acronyms, those such as FBI, where the letters sound as letters, are initialisms.

Online Etymology Dictionary, © 2010 Douglas Harper