The longest word in the English language is ‘pneumonoultramicroscopicsilicovolcanoconiosis’. It has 45 letters and it is a type of lung disease caused by inhaling ash and sand dust.

According to Wikipedia, ‘pneumonoultramicroscopicsilicovolcanoconiosis’ refers to a lung disease contracted from the inhalation of very fine silica particles, specifically from a volcano; medically, it is the same as silicosis.