What Is the Longest Word in English?

The longest word in major English dictionaries is pneumonoultramicroscopicsilicovolcanoconiosis, at 45 letters. Merriam-Webster calls it the longest word entered in most standard English dictionaries and defines it as a lung disease caused by breathing in very fine silicate or quartz dust (Merriam-Webster). It was probably invented to look like a long medical term (Merriam-Webster). A 189,819-letter chemical name is far longer, but Merriam-Webster says that one isn’t technically a word (Merriam-Webster).

The 45-letter record holder

Pneumonoultramicroscopicsilicovolcanoconiosis has 19 syllables. Merriam-Webster says it gets called the longest word in English because it’s the longest word in the largest dictionary, the 20-volume Second Edition of the Oxford English Dictionary (Merriam-Webster).

The OED, Merriam-Webster and Collins all include it:

• The OED has an entry, with its earliest evidence from the New York Herald-Tribune in 1935. It says the word turns up fewer than 0.01 times per million words of modern written English (OED).
• Merriam-Webster enters it in its Medical Dictionary (Merriam-Webster Medical), and editor Emily Brewster said on the dictionary’s Word Matters podcast that it’s the longest word in Merriam-Webster’s unabridged dictionary (Word Matters).
• Collins describes it as an obscure term that “ostensibly” refers to a lung disease caused by silica dust and is sometimes cited as one of the longest words in English (Collins).

The OED’s entry describes it as a word invented, probably by Everett M. Smith (OED). Merriam-Webster says Smith was president of the National Puzzlers’ League, a group of crossword fans, and that the word was meant to imitate a very long medical term. It adds that doctors don’t actually use it (Merriam-Webster).

Other famous long words

The OED and Collins include three other famous long words that Merriam-Webster leaves out.

Word	Letters	Meaning	OED	Merriam-Webster	Collins
supercalifragilisticexpialidocious	34	Nonsense word meaning fantastic or fabulous	Yes	No	Yes
floccinaucinihilipilification	29	The habit of estimating something as worthless	Yes	No	Yes
antidisestablishmentarianism	28	Opposition to disestablishing the Church of England	Yes	No	Yes

Sources for the table: OED entries for supercalifragilisticexpialidocious, floccinaucinihilipilification and antidisestablishmentarianism; Merriam-Webster’s long words list; Collins entries for supercalifragilisticexpialidocious, floccinaucinihilipilification and antidisestablishmentarianism.

Merriam-Webster leaves all three out because they’re rarely or never used in a sentence, only cited as examples of long words. It says it isn’t claiming they aren’t words, just that they don’t meet its criteria for entry (Merriam-Webster).

Antidisestablishmentarianism

The OED defines it as opposition to the disestablishment of the Church of England (or occasionally the Church of Scotland), with evidence from 1891 in the Glasgow Herald. It also notes the word usually appears only where someone gives it as an example of a very long word (OED). Cambridge Dictionary has an entry as well (Cambridge Dictionary).

Merriam-Webster says its files, which go back over 100 years, hold only three citations of the word used with a real meaning. Three citations don’t add up to sustained or widespread use, so it isn’t entered in any Merriam-Webster dictionary (Merriam-Webster).

Floccinaucinihilipilification

The OED labels this one humorous and dates it to 1741. It comes from four Latin words, floccī, naucī, nihilī and pilī, which mean “at a small price” or “at nothing” and were listed together in a well-known rule of the Eton Latin Grammar (OED). Collins says it’s encountered mainly as an example of one of the longest words in English (Collins).

Supercalifragilisticexpialidocious

The OED calls it a nonsense word, originally used especially by children, that expresses excited approval. Its earliest evidence is a 1931 spelling variant in Syracuse University’s Daily Orange, and it says the Walt Disney film Mary Poppins made the word popular in 1964 (OED).

Merriam-Webster’s own statements differ. Its long words list, last updated in September 2025, groups the word with the ones it leaves out (Merriam-Webster). In an older Word Matters episode, though, editor Emily Brewster said the word qualifies under the dictionary’s modern criteria, and Peter Sokolowski said it will probably be entered at some point (Word Matters). Merriam-Webster.com still had no entry for it on October 18, 2026 (Merriam-Webster).

Why chemical names don’t count

Merriam-Webster says most English words longer than about 15 to 20 letters are scientific, meaningless or highly specialized, and are usually built by adding prefixes and suffixes to a base word (Merriam-Webster). The longest string of letters used to describe something is the chemical name for a protein. It starts “methionylthreonylthreonylglutaminyl” and runs to 189,819 letters, fills more than 50 pages, and takes more than three hours to say. Merriam-Webster says it “isn’t technically a word” (Merriam-Webster).

Names like this run into Merriam-Webster’s entry rules. It may reject a word for a general dictionary if all its citations come from highly specialized publications that reflect the jargon of one field (Merriam-Webster). It also says the names of chemicals and other scientific things are part of the rough 1 million estimate of English vocabulary, but many are so far from common use that they don’t, or likely won’t, appear even in an unabridged dictionary (Merriam-Webster). There’s more on how big that vocabulary really is in our guide to how many words are in English.

Long words that are in the dictionary

Merriam-Webster’s long words list doesn’t name a single longest word in everyday use, but it does point to some long words its dictionaries enter:

• acrylonitrile-butadiene-styrene (29 letters plus two hyphens), which Merriam-Webster says is the longest word in its online dictionary (Merriam-Webster). It’s a tough, rigid plastic used for car parts and building materials, also called ABS (Merriam-Webster).
• pseudopseudohypoparathyroidism (30 letters, in the Medical Dictionary), a relatively mild form of pseudohypoparathyroidism and, Merriam-Webster says, the only word in its dictionary with a repeating “pseudo” (Merriam-Webster).
• counterrevolutionaries, deinstitutionalization and otorhinolaryngological, at 22 letters each (Merriam-Webster).

Longest word with no repeated letters

On the Word Matters podcast, Merriam-Webster editor Neil Serven said uncopyrightable is the longest word that doesn’t repeat a letter (Word Matters). It has 15 letters, all different, and Merriam-Webster defines it as not able or allowed to be protected by copyright (Merriam-Webster).