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Pneumonoultramicroscopic silico volcano coneosis (45 letters -- had to misspell it to fit in Urban Dictionary -- merge all of the words togeather); a lung disease caused by breathing in certain particles) is the longest word in any English-language dictionary. (It is also spelled -koniosis.)
On Feb. 23, 1935, the New York Herald-Tribune reported on page 3:

Pneumonoultramicroscopicsilico volcano-koniosis 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 siliceous volcanic dust.
His Pneumonoultramicroscopicsilico volcano koniosis made him cough-up black phlegm and blood.
by Aaron M Smith October 17, 2006
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