pneumonoultramicroscopicsilicovolcanoconiosis is the longest word in an English dictionary at 45 letters. It refers to a lung disease caused by the inhalation of volcanic ash. However, linguists in general reject the word, as it was coined to serve as the longest English word.
The second longest is pseudopseudohypoparathyroidism at 30 letters. And yes, this is a real word.