The longest word in any of the major English language dictionaries is 'pneumonoultramicroscopicsilicovolcanoconiosis', a word that refers to a lung disease contracted from the inhalation of very fine silica particles, specifically from a volcano. Source: http://en.wikipedia.org/wiki/Longest_word_in_English

The previous answer is a scientific name, so it depends. Another is Antidisestablishmentarianism, which isn't disputed as it's neither technical, nor coined. Here's what the word is made up from - anti + dis + establish +ment+ arian + ism Meaning - the movement or ideology that opposes disestablishment (simply not wanting a separation of church and state)