Most people know what an acronym is. But few are as familiar with term initialism, or of an important distinction between the two.
An acronym is an initial abbreviation that can be pronounced as a word, such as NASA or WASP. This term is also used to refer to a series of initials pronounced individually, such as FBI or TGIF, but the technical term is initialism. What’s the BFD (“big, fat deal,” though another word starting with f is sometimes used)? The answer is the.
Because acronyms like NASA are pronounced as words (“na-suh,” in this case), there’s no need to precede them with the definite article: You wouldn’t write “Budget cutbacks hit the NASA hard.” (Though the is essential if NASA is used as an adjective, as in “Budget cutbacks hit the NASA project hard.”)
But initialisms require the: “The FBI announced his capture several hours later.” That’s because the term is pronounced letter by letter: “eff-bee-eye.” (The only usage that omits the definite article is in a headline: “FBI Announces Suspect’s Capture.”)
And when do you use an acronym or an initialism, when do you spell it out, and when do you do both? In more formal writing, the standard approach is to introduce an agency, organization, or some other entity that uses such an abbreviation, or abbreviations of terminology, by spelling out the name on first reference.
If a subsequent reference (called, in publishing, the second reference, no matter how many times it is repeated) appears in proximity — say, the same paragraph — and few, if any, other abbreviations appear in the interim, simply use the abbreviation thereafter.
However, if the next usage is farther away, it’s best to insert the abbreviation in parentheses immediately after the spelled-out reference to establish an association in the reader’s mind when it’s not supported by proximal use of the abbreviation. If the abbreviation is not mentioned again for some time, or it appears only up to a handful of times, spelling it out again (perhaps every time) is a good idea. Depending on the term and the publication, however, the hand-holding parenthesis may be unnecessary.
Many specialized publishers maintain a list of abbreviations familiar to readers of their publications that specifies whether each one requires abbreviation accompanying the spelled-out first reference, or needs to be spelled out at all.
For example, an astronomy magazine is likely to note in its house style guide that NASA does not need to be spelled out, and many other publications do likewise because of the familiarity of the agency’s name in our society. By contrast, less common abbreviations like OEM (original equipment manufacturer) should be spelled out except in a manufacturing-industry publication. As with many usage prescriptions, it’s all about the context.
Oh, and one more point, so to speak: Notice that no acronym or initialism in this post includes periods. They’re generally considered outdated and superfluous. Even two-letter forms like AM and PM, MD and RN, and BC and AD go without, though US stubbornly retains them in many publications.