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. -Wikipedia.org http://en.wikipedia.org/wiki/Pneumonoultramicroscopicsilicovolcanoconiosis

Pneumonoultramicroscopicsilicovolcanoconiosis is the longest standard word I have come across.

http://en.wikipedia.org/wiki/Longest_word_in_English ;