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, causing inflammation in the lungs.' A condition meeting the word's definition is normally called silicosis.
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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 an English language dictionary. It is listed in the current edition of several dictionaries.
http://en.wikipedia.org/wiki/Pneumonoultramicroscopicsilicovolcanoconios...