A long word deliberately coined to be longest word in the English language. It comes in at 45 letters, much longer than supercalifragilisticexpialidocious which is only 34. It is still quite a large number, but pales in comparison to the 45 that pneumonoultramicroscopicsilicovolcanoconiosis holds. It's a lung disease that you get by inhaling smoke or ash, which presumably leads to the "volcano" in the name of the word.
Sir, I'm afraid you've been diagnosed with a severe case of pneumonoultramicroscopicsilicovolcanoconiosis.