The longest word in the world would depend on a lot of things. It depends on if you want to include compound words and also if you want to include scientific words. The longest, non-technical English word is antidisestablishmentarianism. You can find more information here: http://en.wikipedia.org/wiki/Longest_word
The longest word in the English language is pneumonoultramicroscopicsilicovolcanoconiosis. Pneumonoultramicroscopicsilicovolcanoconiosis is the technical term for a lung disease that is contracted through particle inhalation, specifically from a volcano.