Pneumonoultramicroscopicsilicovolcanoconiosis

 Pneumonoultramicroscopicsilicovolcanoconiosis (, also spelled -koniosis) is, according to the Oxford English Dictionary, "a factitious word alleged to mean 'a lung disease caused by the inhalation of very fine silica dust found in volcanoes' but occurring chiefly as an instance of a very long word." It was coined to serve as the longest English word. The more general and widely used term for this condition is pneumoconiosis.

Usage
Pneumonoultramicroscopicsilicovolcanoconiosis is the longest word ever to appear in an English language dictionary. This 45-letter, 19-syllable word, referred to in one linguistics journal as p45, first appeared in the Oxford English Dictionary (OED) in 1936. It is still listed in the current edition of the OED, as well as several current American dictionaries.

Coinage
Word Ways: The Journal of Recreational Linguistics suggests that the word was invented in 1935 by Everett M. Smith, president of the National Puzzlers' League, at its annual meeting. The word figured in the headline for an article published by the New York Herald Tribune on February 23, 1935 titled "Puzzlers Open 103rd Session Here by Recognizing 45-Letter Word":

"Pneumonoultramicroscopicsilicovolcanoconiosis succeeded electrophotomicrographically as the longest word in the English language recognized by the National Puzzlers' League at the opening session of the organization's 103d semi-annual meeting held yesterday at the Hotel New Yorker. The puzzlers explained that the forty-five-letter word is the name of a special form of silicosis caused by ultra-microscopic particles of silica volcanic dust..."

Subsequently, the word was used in a puzzle book, Bedside Manna, after which members of the NPL campaigned to have it included in major dictionaries, eventually succeeding with the 1936 supplement to the OED and Webster's Second.

In popular culture

 * In the Simpsons episode Grampa vs. Sexual Inadequacy, Abe Simpson lists this disease off along with several other old-timey references as to what is troubling Homer in the beginning of the episode.
 * In the Rocket Power episode "Tito's Lucky Shell", Twister, despite being a terrible speller, managed to ace his spelling test, including the bonus word, pneumonoultramicroscopicsilicovolcanoconiosis.


 * In the movie Beerfest, Charlie "Fink" Finklestein (Steve Lemme) refers to one of his students as having discovered a form of pneumonoultramicroscopicsilicovolcanoconiosis.