The longest word in the english dictionary

The word Pneumonoultramicroscopicsilicovolcanoconiosis is defined as "a lung disease caused by the inhalation of very fine silica dust, mostly found in volcanos". It was originally postulated by Everett M. Smith, simply to serve as the longest English word, but has been used in several sources as an approximation of its originally intended definition. The plural of this word, pneumonoultramicroscopicsilicovolcanoconioses is equally long.

It is the longest word ever to appear in an English language dictionary. This 45-letter word, referred to by logologists as "P45", first appeared in the Oxford English Dictionary in 1936.

The name generally used to describe this condition is pneumoconiosis, which is much shorter. Source.