"There is ... an unhappy tendency among amateur etymologists to derive words from the initials of proper names."
-- H.L. Mencken, The American Language
The term acronym only dates from the 1940s and is derived from the Greek akros, meaning point, and onuma, meaning name. In precise usage, the word acronym refers only to terms based on the initial letters of their various elements and read as single words. Initialism refers to terms read as a series of letters. The umbrella term abbreviation is used for acronyms and initialisms.
Thus NATO, pronounced NAY-tow, is an acronym for North Atlantic Treaty Organization, and LASER, pronounced LAY-zer, is an acronym for Light Amplification by Stimulated Emission of Radiation. FBI (Federal Bureau of Investigation) is an initialism. AIDS is an acronym, while HIV is an initialism. URL, when pronounced as three separate letters, "U-R-L," is an initialism for Uniform Resource Locator, but many people pronounce it as "earl" or even "yurl," making it an acronym.
It is a good practice to limit initialisms and acronyms to situations where they add to your message. Be aware that familiarity with initialisms and acronyms is often context sensitive and field dependent. If you use CAD, do you mean Council of Associate Deans or computer-aided design?
- Use initialisms and acronyms in moderation unless your readers are familiar with them.
- Spell out the initialism or acronym on the first use (unless they are undoubtedly widely known, or are never used in their spelled out form) and follow with the abbreviation in parentheses to prepare readers for subsequent use of the abbreviation.
- Consider rewriting documents that are loaded with acronyms.
- Do not italicize acronyms or initialisms even if they are the official title of a printed piece.
One of the most often asked questions about grammar has to do with the choice of articles - a, an, the - to precede an initialism or acronym. Do we say an FBI agent or a FBI agent? Although "F" is obviously a consonant and we would generally precede any word that begins with "F" with a, we precede FBI with anbecause the first sound we make when we say FBI is not an "f-sound," it is an "eff-sound."
Thus we say we're going to a PTO meeting where an NCO will address us. We say we saw aUFO because, although the abbreviation begins with a "U," we pronounce the "U" as if it were spelled "yoo."
Whether we say an URL or a URL depends on whether we pronounce it as "earl" or as "u-r-l." Acronyms, except when used adjectivally, are rarely preceded by a, an, or the("member nations of NATO").
Adapted from: The Chicago Manual of Style and other sources.