What is the World’s Longest Word?

The longest word in the world is the full chemical name the protein titin with a total of 189,819 letters: Methionylthreonylthreonylglutaminylarginyl...isoleucine. There is some dispute as to whether it is a real word or not, however. The longest word to officially appear in a dictionary is Pneumonoultramicroscopicsilicovolcanoconiosis. It is the name of a lung disease that is caused by inhalation of mineral dust.