Longest words

The longest English word is pneumonoultramicroscopicsilicovolcanoconiosis (43 letters), which is a disease caused by a prolonged exposure to fine siliceous dust.

The second, third, and forth longest words are as follows:

While pneumonoultramicroscopicsilicovolcanoconiosis is the longest word you can find in the English dictionary, some people disagree that it is truly the longest English word. For instance, chemists occasionally use molecule names with many more letters. While you will not find these long chemical names in the dictionary, they are valid scientific terms, which represent the structure of a complex molecule by a very long compound word. The longest of chemical terms is methionylthreonylthreonylglutaminylarginyl...isoleucine, which contains 189,819 letters. This monster word descibes the strucutre of titin, which is the largest known protein.

Another good candidate for the longest word is lopadotemachoselachogaleokranioleipsanodrimhypotrimmatosilphioparaomelitokatakechymenokichlepikossyphophattoperisteralektryonoptekephalliokigklopeleiolagoiosiraiobaphetraganopterygon, which is the English translitiration of a fictional dish once mentioned by Aristophanes', an Ancient Greek writer.

Finally, some people claim that the longest English word is smiles because it has a whole mile between its first and last letters.

You may learn more about long English words from the related Wikipedia article.