What Is the Difference Between an Acronym and an Initialism?

Do you know the difference between an acronym and an initialism? They are both types of abbreviations, and both terms refer to shortened forms made up of a set of capitalized letters (rather than a shortened word, such as Dr.). However, they have different meanings and work in different ways. To help you nail down the details of these two terms, we’ll take a close look at the difference between acronyms and initialisms.

What Are Acronyms and Initialisms?

Acronyms and initialisms are both abbreviations that are formed from the first letter of each word in a full phrase. But the key difference hinges on how the result is presented:

• Acronyms are pronounced as one word. For example, we often shorten President of the United States to POTUS. 
• Initialisms are pronounced as individual letters. For example, we say F-B-I to abbreviate the Federal Bureau of Investigation. 

It’s worth noting that, in US English, the term “acronym” is sometimes used more broadly to refer to any abbreviation formed from the first letters of a phrase, whether it’s pronounced as a word or as individual letters. However, for clarity, we will maintain the distinction between acronyms (pronounced as words) and initialisms (pronounced letter by letter).

The table below gives some examples of both.

Acronyms	Initialisms
NATO (North Atlantic Treaty Organization)	BBC (British Broadcasting Corporation)
NASA (National Aeronautics and Space Administration)	FBI (Federal Bureau of Investigation)
UNESCO (United Nations Educational, Scientific and Cultural Organization)	IT (Information Technology)
GIF (Graphics Interchange Format)	PDF (Portable Document Format)
PIN (personal identification number)	RSVP (répondez s’il vous plaît)

How to Introduce Acronyms and Initialisms

An acronym or initialism should always be introduced the first time the term is used. However, there are a couple of exceptions to this rule! But we’ll come to those in a moment.

When the full term is first mentioned in a text, include the acronym or initialism in parentheses directly after it: 

Banks use magnetic-ink character recognition (MICR) technology to verify the authenticity of documents and speed up processing.

After this, the abbreviation should be used by itself for all subsequent mentions. 

So, what does this mean for a proofreader? If you see an acronym or initialism used without being introduced:

• You can fix the error in the text if you are absolutely certain what term the client has abbreviated (e.g., it’s been used elsewhere in the text) 
• You can leave a comment asking the client to introduce the term in full, with the abbreviation in parentheses

When an Introduction Isn’t Necessary

There are some situations when you don’t need to (or simply should not) use the full term followed by the abbreviation in parentheses.

The first, and most common, reason is when the acronym or initialism is ubiquitous or certain to be familiar to the reader. For example, it is more than likely that most people will know what TV stands for, so you don’t have to spell out television the first time this word is used.

Some other abbreviations would be commonly understood only by those in certain locations, so how you present them may depend on which dialect the client is writing in and where the content is going to be published. Here are some examples:

• As the federal tax service, the IRS would be widely recognized by those in the US so may not need to be introduced in all instances for a US audience.
• The BBC is a public service broadcaster that would be instantly recognizable in the UK.
• With 32 member countries, NATO would be known by many readers around the world.

The second reason not to introduce an acronym or initialism is if the style guide says not to. For example, AP style recommends simply writing out the full term on first use, and then using the shortened form in subsequent mentions – no parentheses in sight. 

And the third reason why you would not introduce an abbreviation is when the full term is only used once in a text. In this case, it would be redundant to add the acronym or initialism in parentheses since it wouldn’t have a chance to be used again!

When to Use Articles with Acronyms and Initialisms

Whether you need to use an article (either definite or indefinite) before an acronym or initialism depends on pronunciation and context. This can be one of the trickier points to judge, but there are some key rules.

Acronyms, which are pronounced as words, generally do not take a definite article when they are proper nouns used alone:

NASA launched a new satellite last month.

However, when an acronym modifies a noun or is used as an adjective, an article may be appropriate:

The NASA project is underway.

Initialisms, on the other hand, more often require a definite article. This is because the abbreviation is not treated as a proper noun by the reader:

The CEO briefed the board about the FBI’s involvement.

Initialisms that are used as general terms – such as those for diseases, chemicals, or organizations – don’t generally take an article when used as subjects:

DNA carries the genetic instructions for living organisms.

In contrast, when the initialism is treated as a specific entity or when it qualifies another noun, an article should be used:

The DNA was sent to the research team.

The DNA sequence was then analyzed for mutations.

When you are choosing between the indefinite articles a and an, consider how the abbreviation is pronounced. If the first letter is spoken as a vowel sound (e.g., F as eff), then use an. And use a if the initialism is pronounced with a consonant sound (e.g., U as you):

An FBI file was recovered during the raid.

A UNESCO representative attended the conference.

An HR manager will review your application.

This last one can trip some people up! The official pronunciation of the letter H is aitch, so it should be preceded by an. However, certain regional accents mean that some people may pronounce it as haitch, beginning with a consonant sound. So be sure to carefully check articles before abbreviations.

How to Punctuate Acronyms and Initialisms

As a rule of thumb, acronyms and initialisms do not have periods between the letters in UK English, but they often do include periods in US English. 

However, over time, punctuated abbreviations (e.g., C.E.O.) have fallen out of vogue in the US as well. An interesting combination of rules occurs with the terms US and USA.

In US English, the general rule is to use periods when you use U.S. or U.S.A. as a noun: 

I am visiting the U.S. next week. 

But, when you use US or USA as an adjective or a modifier, periods are unnecessary: 

Amazon, a US company, is one of the highest grossing companies in history. 

Of course, whether to punctuate very much depends on the style guide you are using and the client’s preferences. If you are uncertain about whether a particular acronym or initialism should have periods between the letters, it is always advisable to check the client’s style guide (or the preferred dictionary when it comes to common terms) and then adapt the style accordingly.

How to Capitalize Acronyms and Initialisms

Generally, we use all caps to write most acronyms and initialisms. However, a few specific initialisms should be written in lowercase, and acronyms in some contexts only take a capital on the first letter. Let’s take a closer look at some examples.

Capitalizing Initialisms

We almost always capitalize initialisms. However, there are a couple of exceptions:

• The terms e.g. and i.e. are written in lowercase (apart from when presented in title case or on the rare occasions when they start a sentence).
• Some terms combine uppercase and lowercase letters in the abbreviation as standard, for example, the proprietary system iOS (meaning iPhone Operating System).

Capitalizing Acronyms

In the same way, most acronyms are fully capitalized. But, again, there are a few situations where this is not the case! 

As with certain initialisms, some acronyms use a mix of letters as their typical style, such as the file format LaTeX and the anti-bot verification system reCAPTCHA. 

Another example is with those acronyms that have become so well known they are now words in their own right. For many of these, it’s no longer common knowledge that they were acronyms to begin with. The table below gives some examples (we’ve capitalized the key words to show the evolution, but capitalization of the full phrases can vary).

Common Word	Original Full Term
Laser	Light Amplification by Stimulated Emission of Radiation
Radar	Radio Detection and Ranging
Scuba	Self-Contained Underwater Breathing Apparatus
Taser	Thomas A. Smith Electric Rifle

Capitalization can also be a matter of stylistic preference. For example, while PIN is common, The Guardian newspaper recommends pin or pin number and the BBC recommends Pin number (or Pin code). Although number is redundant here, it is commonly used, so these news sites still include it in their style guides.

Furthermore, some styles – including The Guardian – choose to capitalize acronyms as proper nouns. This means writing Nato, Nasa, or Unicef with only the first letter capitalized. This approach mirrors the treatment of ordinary nouns and reflects their pronunciation as single words.

How to Make Acronyms and Initialisms Plural

You can pluralize acronyms and initialisms in the same way as any other noun. However, there are some fairly common errors associated with making abbreviations plural, so it’s something to be particularly aware of. Here’s how it looks in action:

Singular: It is vital to meet or exceed each key performance indicator (KPI).

Plural: The team successfully achieved all KPIs.

The first common error is failing to make the term plural:

The team successfully achieved all KPI. ✘

Another is to incorrectly add an apostrophe before the letter -s at the end:

The team successfully achieved all KPI’s. ✘

And another error that can occur is capitalizing the -s to match the other letters:

The team successfully achieved all KPIS. ✘

In some cases, it may be best to slightly reword the sentence to make the term singular, if it helps to avoid an awkward construction. Otherwise, simply add a lowercase -s to make an abbreviation plural.

How to Avoid Errors with Acronyms and Initialisms

Aside from dealing with any contradictions to the guidelines we’ve covered so far, there are a couple of common issues it’s best to be on the lookout for.

Alphabet Soup

When several acronyms or initialisms are used in close succession, it can overwhelm the reader and affect clarity. This is known as “alphabet soup.” For example:

The CEO of the NGO met with the UN, WHO, and IMF to discuss GDP impacts on LMICs.

Even though each term may be familiar to the audience, the sentence becomes difficult to process when so many abbreviations are grouped together.

As a proofreader, look out for sentences packed with acronyms and initialisms and consider suggesting edits for clarity or recommending adding a list of abbreviations at the start of the document if usage is especially heavy.

Redundancies

Redundancies happen when a word from the acronym is used after the abbreviation. You may recall using the most common one yourself!

I forgot my PIN number ✘

I forgot my PIN ✔

As we mentioned earlier, the N stands for number, so it is redundant to say it again. Here are some more common examples of abbreviation redundancies:

Where is the nearest ATM machine? ✘

Where is the nearest ATM? ✔

View the readout on the LCD display screen. ✘

View the readout on the LCD screen. ✔

We need to check the VIN number before selling the car ✘

We need to check the VIN before selling the car ✔

Please RSVP soon. ✘

RSVP. (Or Please respond soon.) ✔

Your main goal as a proofreader should always be to ensure clarity and consistency. As such, you can follow the rules we’ve set out in this post to tidy any abbreviations you come across. However, there may be exceptions! If you are unsure about anything, your first port of call should always be to check the applicable style guide. If no guidance is available, you can leave a comment for the client if necessary.

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Updated on April 14, 2025