Answer:
Pneumonoultramicroscopicsilicovolcanoconiosis is a word coined by the then president of the National Puzzlers' League, Everett M. Smith, as a synonym for the disease known as silicosis.
Silicosis is a form of occupational lung disease caused by inhalation of crystalline silica dust, and is marked by inflammation and scarring in the form of nodular lesions in the upper lobes of the lungs. It is a type of pneumoconiosis.
The word pneumonoultramicroscopicsilicovolcanoconiosis was created in the 1950s.