The longest English word

Pneumonoultramicroscopicsilicovolcanoconiosis 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, causing inflammation in the lungs.' A condition meeting the word's definition is normally called silicosis..

It occurs chiefly as an instance of a very long word. The 45-letter word was coined to serve as the longest English word and is the longest word ever to appear in anEnglish language dictionary. It is listed in the current edition of several dictionaries.

This 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 103d 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 103rd 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.

This 45-letter word, referred to as P45, first appeared in the 1939 supplement to the Merriam-Webster New International Dictionary, Second Edition.