Abbreviations and Acronyms: Rules, Differences, and Examples

Definitions: Abbreviation, Acronym, and Initialism

These three terms are often used interchangeably, but they have distinct meanings. Understanding the differences will help you use them correctly in your writing and look them up properly in a dictionary.

Abbreviation

An abbreviation is any shortened form of a word or phrase. It is the broadest category, encompassing acronyms and initialisms as subtypes. Examples include:

• Dr. (Doctor)
• etc. (et cetera)
• govt. (government)
• approx. (approximately)
• dept. (department)

Acronym

An acronym is an abbreviation formed from the initial letters of a phrase and pronounced as a single word:

• NASA (National Aeronautics and Space Administration) — pronounced "nasa"
• NATO (North Atlantic Treaty Organization) — pronounced "nay-toe"
• UNICEF (United Nations International Children's Emergency Fund)
• scuba (self-contained underwater breathing apparatus)
• laser (light amplification by stimulated emission of radiation)

Initialism

An initialism is an abbreviation formed from initial letters but pronounced letter by letter:

• FBI (Federal Bureau of Investigation) — pronounced "eff-bee-eye"
• HTML (Hypertext Markup Language)
• CEO (Chief Executive Officer)
• USA (United States of America)
• ATM (Automated Teller Machine)

Key Differences

In everyday usage, many people call initialisms "acronyms." While technically imprecise, this is widely understood and accepted in informal contexts. In formal writing, however, precision matters.

Introducing Abbreviations and Acronyms in Writing

The standard practice in formal and academic writing is to spell out the full term on first use, followed by the abbreviation in parentheses:

The World Health Organization (WHO) issued new guidelines. The WHO report emphasized the importance of vaccination.

After the first introduction, use the abbreviation consistently throughout the document. Do not alternate between the full term and the abbreviation.

Exceptions

Very common abbreviations that your audience will immediately recognize do not need to be spelled out:

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

The test is: will every reader know what this means without an explanation? If yes, skip the spell-out.

Periods in Abbreviations

Whether to use periods in abbreviations is one of the most common questions in English punctuation. The rules vary by type and style guide:

Lowercase Abbreviations

Abbreviations of lowercase words typically retain periods: e.g., i.e., etc., vs.

Uppercase Abbreviations (Acronyms and Initialisms)

Modern practice strongly favors omitting periods from all-caps abbreviations:

• Modern (preferred): NASA, FBI, NATO, UK, EU
• Traditional: N.A.S.A., F.B.I., N.A.T.O.

Personal Titles

American English uses periods after courtesy titles; British English typically omits them when the abbreviation ends with the same letter as the full word (Doctor → Dr).

US States

The two-letter postal abbreviations for US states (CA, NY, TX) do not take periods. Older traditional abbreviations (Calif., N.Y., Tex.) do.

Articles (A vs. An) Before Acronyms

Whether to use "a" or "an" before an acronym depends on the sound, not the letter:

• An FBI agent (F starts with a vowel sound: "eff")
• A NASA engineer (N starts with a consonant sound: "en")
• An HTML document (H starts with a vowel sound: "aitch")
• A UNICEF report (U starts with a consonant sound: "you")
• An MBA degree (M starts with a vowel sound: "em")

Forming Plurals of Abbreviations

To form the plural of an abbreviation, simply add a lowercase s without an apostrophe:

• CEOs (not CEO's)
• URLs (not URL's)
• PhDs (not PhD's)
• DVDs (not DVD's)

Using an apostrophe for plurals is one of the most common punctuation errors. The apostrophe indicates possession, not plurality. The only time an apostrophe might be used is when the abbreviation itself contains periods and adding "s" alone could cause confusion, but this situation is rare in modern writing.

Possessives of Abbreviations

To form the possessive of an abbreviation, add 's:

• the CEO's decision
• NASA's latest mission
• the FBI's investigation

For plural possessives, add the s first, then the apostrophe:

• the CEOs' combined salaries

Common Abbreviations in English

Latin Abbreviations

Several Latin abbreviations are commonly used in English writing. Understanding their meaning prevents misuse:

The most commonly confused pair is e.g. and i.e.:

e.g. = "for example" → Citrus fruits (e.g., oranges, lemons, limes) are rich in vitamin C.

i.e. = "that is" → The capital of France (i.e., Paris) hosted the Olympics.

Style Guide Recommendations

Common Errors

Error 1: Confusing e.g. and i.e.

Remember: e.g. gives examples, i.e. gives a definition or restatement.

Error 2: Using an Apostrophe for Plurals

Incorrect: She earned two PhD's.

Correct: She earned two PhDs.

Error 3: Redundancy with Acronyms

• Incorrect: ATM machine (Automated Teller Machine machine)
• Incorrect: PIN number (Personal Identification Number number)
• Incorrect: LCD display (Liquid Crystal Display display)

Error 4: Not Spelling Out on First Use

Never assume your reader knows what an abbreviation means. Always spell it out the first time unless it's universally known.

Summary and Key Takeaways

• Abbreviations are any shortened forms; acronyms are pronounced as words; initialisms are pronounced letter by letter.
• Spell out on first use with the abbreviation in parentheses.
• Modern style omits periods from most uppercase abbreviations (NASA, FBI, CEO).
• Use "a" or "an" based on the sound, not the letter.
• Form plurals with lowercase s, no apostrophe: CEOs, PDFs, URLs.
• e.g. means "for example"; i.e. means "that is."
• Avoid redundant acronyms like "ATM machine."

For related topics, see our guides to capitalization rules, punctuation marks, and formal vs. informal English.