An acronym is a series of letters taken from a set of words that is pronounced as a word (for example, “NATO” for the North American Trade Organization). An initialism is a series of letters taken from a set of words where each letter is pronounced individually (for example, “DEA” for the Drug Enforcement Agency).
Using acronyms and initialisms can quicken your pacing and help you avoid repetition of long phrases. In nearly every case, however, you should spell out the full name, title or phrase the first time you refer to it in text, followed immediately by the acronym or initialism in parentheses.
You should then use the acronym or initialism for each subsequent use. If your text contains only one reference, spell out the full name, and do not note the acronym or initialism in parentheses. For the sake of brevity, headlines may use common but unidentified acronyms and initialisms, but make sure to spell out the full term upon first reference in text.
These conventions aren’t always appropriate, however. If the next appearance of an uncommon acronym or initialism comes much later in the piece — such that no reader could be expected to remember it — it’s often best to spell it out again. At the same time, a few acronyms and initialisms are more commonly understood than the full titles they represent (such as HIV and NCAA), and spelling out their meanings can clog up the flow of a pleasant sentence.
Right: The National Institutes of Health (NIH) awarded the grant to the research group. The NIH funded only three such centers in the nation.
OK: Although she won a prized internship at NASA, she couldn’t afford the move to Houston.
Wrong: URSA’s five-year research project is funded by the ARL.