"written word formed from the first letters, in order, of other words in a name or phrase," 1957, from initial (n.) + -ism. The word was used earlier in a sense "group of initial letters of an author's name (rather than the full name) on a publication" (1868). An earlier term for what we now call an initialism was alphabetic abbreviation (1907).
The distinction from acronym is not universally agreed-upon; but in general, cases such as NATO, where the letters always are sounded as a word, are regarded as acronyms, those such as FBI, where the letters sound as letters, are initial-words or initialisms. The use of acronym in entries in this dictionary that are technically initialisms is a deliberate error, because many people will search all such words using "acronym."