Pneumonoultramicroscopicsilicovolcanoconiosis is, according to the Oxford English Dictionary, "a lung disease caused by the inhalation of very fine silica dust, causing inflammation in the lungs."
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 editions of several dictionaries.
