Pneumonoultra-microscopicsili-covolcanokoniosis
: a pneumoconiosis caused by the inhalation of very fine silicate or quartz dust and occurring especially in miners
Example:
"Miss Carol Lee Sutherland's seventh-grade class at J. E. B. Stuart School is learning to spell a 45-letter word. It's pneumonoultramicroscopicsilicovolcanokoniosis. It's a lung disease." — The Virginian-Pilot, 23 March 1962
About the Word:
Weighing in at a hefty 19 syllables and 45 letters, pneumonoultramicroscopicsilicovolcanoconiosis is often referred to as the longest word in the English language, on the basis that it is the longest word found in the largest dictionary (the 20-volume Second Edition of the Oxford English Dictionary). It is often referred to as an 'invented word,' which may be a bit confusing, since all words had to have been invented at some point of other. The difference here is that pneumonoultramicroscopicsilicovolcanoconiosis was thought to have been invented by the president of the National Puzzlers' League (a group of crossword puzzle aficionados), one Everett Smith, and was meant to imitate a very long medical term. You can say it's the longest word in the language if you want, but you should know that doctors don't actually use this word, and will probably laugh at you if you do.
