Definition from Wiktionary, the free dictionary
acronym (plural acronyms)
- An abbreviation formed by (usually initial) letters taken from a word or series of words, that is itself pronounced as a word, such as RAM, radar, or scuba; sometimes contrasted with initialism.
- A pronounceable word formed from the beginnings (letter or syllable) of other words and thus representing the phrase so formed, e.g. Benelux = the countries Belgium, Netherlands and Luxembourg considered as a political or economic whole.
- Any abbreviation so formed, regardless of pronunciation, such as TNT, IBM, or XML.
Usage notes 
- The original definition was "term for words made from the initial letters or syllables of other words", indicating that it should be pronounceable as a word, e.g. snafu, comsat.
- The third sense is often criticized by commentators who prefer the term initialism for abbreviations that are not pronounced like an ordinary word.
- Acronyms are generally written with all letters in upper case. Some acronyms are treated as words in their own right and are written in lower case (such as scuba or radar).
- In American English, the general rule is that acronyms of more than four letters are written in lower case (e.g., laser) while those with four or fewer letters are written in all caps (e.g., NATO). (In European usage the latter example is sometimes written "Nato.")
- Older usage required that each letter be followed by a full stop (period). Current usage tends to omit full stops.
- In recent years, the use of mixed capitalization in acronyms has become more common in two circumstances. The first occurs when the words contain articles that are not capitalized when spelled out in title case – for instance, Field of Regard initialized as "FoR." (This can be helpful in preventing confusion among acronyms spelled the same way to represent different meanings.) The other circumstance occurs when the writer is attempting to make an acronym pronounceable when it otherwise would not be by including more than just initial letters – for instance, "RaZR".
- Acronyms also exist which, for convenience's sake, are pronounced with interstitial sounds, usually vowels, placed among the initials to make pronunciation easier. An example is WSDL (web services description language) pronounced wizdul.
- ^ American Speech Vol. 18, No. 2. (Apr., 1943), page 142
- ^ http://www.ibm.com/developerworks/library/lol/acronym.html
word formed by initial letters
any abbreviation formed by initial letters