pneumonoultramicroscopicsilicovolcanoconiosis

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’.”
It was coined to serve as the longest English word and is the longest word ever to appear in an English language dictionary.