The longest word in English dictionary is pneumonoultramicroscopicsilicovolcanoconiosis, a 45-letter medical term for a dust-induced lung disease. While even longer technical strings exist in chemistry, they aren’t included in standard dictionaries. Below is a ranked list of the longest English words—real vocabulary entries you can actually look up—complete with definitions and sample sentences. Whether you’re a logophile, a trivia buff, or just curious, these entries showcase the extreme possibilities of English word formation.

1. Pneumonoultramicroscopicsilicovolcanoconiosis – 45 letters

• Part of speech: noun

Definition: The longest word. A lung disease caused by inhaling very fine silicate or quartz dust; essentially an elaborate synonym for silicosis.
Example: The coal miner’s health screening ruled out pneumonoultramicroscopicsilicovolcanoconiosis, much to everyone’s relief.

2. Pseudopseudohypoparathyroidism – 30 letters

• Part of speech: noun

Definition: A relatively mild hereditary disorder that mimics the hormonal abnormalities of pseudohypoparathyroidism.
Example: The patient’s test results confirmed pseudopseudohypoparathyroidism, a tongue-twisting diagnosis for a rare condition.

3. Floccinaucinihilipilification – 29 letters

• Part of speech: noun

Definition: The act of estimating something as worthless.
Example: With a touch of floccinaucinihilipilification, the critics dismissed the new art installation as trivial.

4. Methylenedioxymethamphetamine – 29 letters

• Part of speech: noun

Definition: The full chemical name for MDMA, a synthetic amphetamine with mood-enhancing and hallucinogenic effects.
Example: Law enforcement seized a large batch of methylenedioxymethamphetamine, better known on the streets as ecstasy.

5. Antidisestablishmentarianism – 28 letters

• Part of speech: noun 

Definition: Opposition to disestablishing a state church—historically, the movement to keep the Church of England as the state church.
Example: In a history discussion, a student jokingly cited antidisestablishmentarianism to describe resistance to church-state separation.

6. Honorificabilitudinitatibus – 27 letters

• Part of speech: noun

Definition: The state of being able to achieve honors; honorableness.
Example: The scholar pointed out that honorificabilitudinitatibus appears in Shakespeare’s play as a playful boast about honor.

7. Electroencephalographically – 27 letters

• Part of speech: adverb

Definition: By means of recording brain activity with an electroencephalograph (EEG).
Example: The researchers electroencephalographically monitored the patient’s sleep stages to observe brain-wave patterns.

8. Radioimmunoelectrophoresis – 26 letters

• Part of speech: noun

Definition: A lab technique that combines immunoelectrophoresis with radioactive labels to separate and identify proteins.
Example: Using radioimmunoelectrophoresis, the lab separated the proteins with a radioactive tracer.

9. Thyroparathyroidectomized – 25 letters

• Part of speech: adjective

Definition: Having had surgical removal of the thyroid and parathyroid glands.
Example: Calcium levels in the thyroparathyroidectomized patient must be monitored closely after surgery.

10. Immunoelectrophoretically – 25 letters

• Part of speech: adverb

Definition: By the method of immunoelectrophoresis (a technique for separating substances with antibodies and an electric field).
Example: The samples were immunoelectrophoretically analyzed to detect specific proteins.

11. Laryngotracheobronchitis – 24 letters

• Part of speech: noun

Definition: Inflammation of the larynx, trachea, and bronchi—commonly associated with croup.
Example: The child was diagnosed with laryngotracheobronchitis, explaining her coughing and breathing difficulties.

12. Dichlorodifluoromethane – 23 letters

• Part of speech: noun

Definition: A colorless, nonflammable gas once used as a refrigerant and aerosol propellant (also called Freon-12).
Example: Older air-conditioning units contained dichlorodifluoromethane as a coolant until it was phased out.

13. Hydrochlorofluorocarbon – 23 letters

• Part of speech: noun

Definition: Any of a class of compounds (HCFCs) developed to replace more harmful chlorofluorocarbons.
Example: The refrigerant R-22 is a hydrochlorofluorocarbon now being replaced by more eco-friendly alternatives.

14. Otorhinolaryngological – 22 letters

• Part of speech: adjective

Definition: Relating to the medical specialty dealing with the ear, nose, and throat (ENT).
Example: The hospital’s otorhinolaryngological department treated hearing loss and sinus issues.

15. Counterrevolutionaries – 22 letters

• Part of speech: noun (plural)

Definition: Individuals who participate in a movement aimed at overthrowing a government established by a previous revolution.
Example: The novel portrayed the rebels as counterrevolutionaries seeking to restore the old order.

16. Deinstitutionalization – 22 letters

• Part of speech: noun

Definition: The process of releasing institutionalized individuals into community care or reforming institutions to reduce confinement.
Example: The shift toward deinstitutionalization led to community-based support for many psychiatric patients.

17. Incomprehensibilities – 21 letters

• Part of speech: noun (plural)

Definition: Things that are impossible or extremely difficult to understand.
Example: The technical jargon filled the contract with incomprehensibilities for lay readers.

18. Pseudohermaphroditism – 21 letters

• Part of speech: noun

Definition: A condition where an individual’s gonads do not match the appearance of external genitalia; an older medical term often replaced by “intersex.”
Example: The textbook discussed pseudohermaphroditism in cases where chromosomal sex and outward anatomy differed.

19. Psychoneuroimmunology – 21 letters

• Part of speech: noun

Definition: The study of how psychological factors and the nervous system influence immune function.
Example: Research in psychoneuroimmunology suggests chronic stress can weaken immunity.

20. Uncharacteristically – 20 letters

• Part of speech: adverb

Definition: In a manner not typical or characteristic of someone or something.
Example: He was uncharacteristically quiet during the meeting, which concerned his usually talkative colleagues.

There you have it!

These 20 lengthy entries demonstrate the astonishing elasticity of the longest words in English, from medical terminology to playful Latin-based constructs. While most aren’t part of daily conversation—except perhaps uncharacteristically—they highlight how prefixes, suffixes, and word roots can stretch vocabulary to impressive lengths. Whether used for precise scientific description or linguistic fun, these heavyweight words remind us that English can be as short as “yes” or as long as pneumonoultramicroscopicsilicovolcanoconiosis—and still be perfectly at home in a dictionary.

Sources: Major dictionary references including Merriam-Webster, Oxford English Dictionary, Dictionary.com, and specialized medical dictionaries.

FAQ

What is the longest word in the English?

The longest widely accepted dictionary word is pneumonoultramicroscopicsilicovolcanoconiosis, a 45-letter term for a lung disease caused by inhaling fine silica dust.

How many letters does the longest English word have?

It has 45 letters—making it nearly twice as long as most other entries on the longest-word list.

Are ultra-long chemical names counted as English words?

No. Full chemical names (such as the 189,000-letter protein name for titin) are considered technical strings, not dictionary words, so they’re excluded from standard vocabulary lists.

Which long word is actually used in everyday conversation?

Uncharacteristically (20 letters) is the longest word on the list that you might realistically hear in daily speech.

Why do many long English words come from medicine or science?

Scientific terminology often combines multiple Latin or Greek roots to convey precise meaning, resulting in very long compound words.