Abbreviations and Acronyms: Rules, Differences, and Examples

What Each Term Actually Means

People swap these three labels around as if they mean the same thing. They don't. Getting the distinction straight makes your writing sharper and helps when you look a term up in a dictionary.

Abbreviation

The word abbreviation is the umbrella. Any shortened form of a longer word or phrase counts, including both acronyms and initialisms. Some quick examples:

• Blvd. (Boulevard)
• Prof. (Professor)
• vol. (volume)
• misc. (miscellaneous)
• corp. (corporation)

Acronym

An acronym takes the first letter (or first few letters) of each word in a phrase and glues them together into something you pronounce like an ordinary word:

• radar (radio detection and ranging)
• POTUS (President of the United States) — said "poe-tus"
• SWAT (Special Weapons and Tactics) — said "swat"
• AWOL (absent without leave)
• GIF (graphics interchange format)

Initialism

An initialism also uses first letters, but you say each letter individually rather than blending them:

• IRS (Internal Revenue Service) — said "eye-are-ess"
• DVD (Digital Versatile Disc)
• UFO (Unidentified Flying Object)
• PDF (Portable Document Format)
• BBC (British Broadcasting Corporation)

How They Actually Differ

In casual speech, most people lump initialisms under the label "acronym." That's technically sloppy, but nobody is going to correct you at a dinner party. For formal writing, though, the distinction is worth respecting.

Bringing Them Into Your Writing

The convention in academic and professional prose is simple: write out the full name the first time it appears, then drop the short form in parentheses right after it:

The Centers for Disease Control and Prevention (CDC) updated its travel advisory. The CDC now recommends testing before departure.

Once you've introduced the short form, stick with it for the rest of the piece. Bouncing back and forth between the full phrase and the abbreviation just distracts the reader.

Exceptions

You don't need to spell out terms that every reader already knows cold:

• DNA, HIV, AIDS
• USA, UK, EU, UN
• PDF, URL, HTML
• CEO, CFO

A useful gut check: if a general reader would pause and wonder what the letters stand for, spell it out. If not, leave it alone.

Do You Need Periods?

Few questions about English punctuation come up more often than this one. The answer depends on the type of abbreviation and which style guide you're following:

Lowercase Shortenings

When the abbreviation is built from lowercase letters, periods usually stay: i.e., e.g., vs., etc.

All-Caps Abbreviations

Contemporary usage drops the dots from all-caps forms almost every time:

• Old-fashioned: F.B.I., N.A.S.A., N.A.T.O.
• Today's norm: FBI, NASA, NATO, EU, UK

Titles Before Names

American English puts a period after courtesy titles. British English usually leaves it off when the shortened form ends on the same letter as the complete word (so Doctor becomes Dr, with no dot).

State Abbreviations

The two-letter postal codes (TX, CA, NY) never take periods. The older, longer versions (Tex., Calif., N.Y.) do.

Choosing A or An

What comes before an acronym is decided by sound, not the letter on the page:

• A UNESCO delegate (U sounds like "you," a consonant sound)
• An SAT score (S sounds like "ess," a vowel sound)
• A NATO member (NATO is said "nay-toe," a consonant sound)
• An XML file (X sounds like "ex," a vowel sound)
• An LLM course (L sounds like "el," a vowel sound)

Making Them Plural

Pluralizing an abbreviation is easy: tack on a lowercase s and leave the apostrophe out of it:

• SUVs (not SUV's)
• MP3s (not MP3's)
• MBAs (not MBA's)
• RVs (not RV's)

Sticking an apostrophe in there is one of the most frequent punctuation slip-ups. Apostrophes mark possession, not more-than-one. The only place an apostrophe might earn its keep is with older period-laden forms where adding "s" alone would look confusing — but that's a corner case you rarely hit in modern text.

Showing Possession

To turn an abbreviation into a possessive, add 's:

• the CDC's guidance
• the FBI's files
• the UN's resolution

If the abbreviation is already plural, add the s first, then the apostrophe:

• the CFOs' joint statement

Everyday Abbreviations You Should Know

Latin Shortcuts Still in Use

A handful of Latin holdovers still show up regularly in English. Knowing what they actually mean keeps you from misusing them:

The pair that trips writers up the most is e.g. and i.e.:

e.g. = "for example" → Leafy greens (e.g., spinach, kale, arugula) belong in most diets.

i.e. = "that is" → My youngest sibling (i.e., Marco) just started college.

What the Major Style Guides Say

Mistakes People Make

Mixing Up e.g. and i.e.

Quick mental shortcut: e.g. offers examples, i.e. offers a definition or rewording.

Using an Apostrophe to Pluralize

Wrong: The company hired three CPA's last quarter.

Right: The company hired three CPAs last quarter.

Saying the Word Twice

• Wrong: PIN number (Personal Identification Number number)
• Wrong: ATM machine (Automated Teller Machine machine)
• Wrong: HIV virus (Human Immunodeficiency Virus virus)

Skipping the Spell-Out

Don't assume your audience recognizes every acronym you do. Spell it out on first appearance unless the term is genuinely universal.

Quick Recap

• Keep the hierarchy straight: abbreviation is the umbrella, acronym is said as a word, initialism is said letter by letter.
• Expand on first appearance, then use the short form for the rest of the piece.
• Drop the periods from all-caps forms (NASA, FBI, CEO) in modern usage.
• Pick a or an based on the sound that starts the abbreviation, not the written letter.
• Plurals take a plain lowercase s — no apostrophe.
• Remember that e.g. means "for example" while i.e. means "that is."
• Watch for doubled-up terms like "PIN number" or "ATM machine."

For related reading, take a look at our guides to capitalization rules, punctuation marks, and formal vs. informal English.