I thought it was supercalifragilisticexpalidosious.
something like that, just kidding.
Officially the longest word is 'floccinaucinihilipilification' at 29 characters, meaning 'the act of estimating as worthless'.
Then there's 'antidisestablishmentarianism' at 28 letters, meaning 'opposition to the disestablishment of the Church of England'. It is often considered the longest word as it has an actual meaning instead of being created just to be long.
Unofficially the longest word is 'pneumonoultramicroscopicsilicovolcanoco... at 45 characters, meaning 'a lung disease'. It was created solely for the purpose of being the longest word, however, it does appear in a few dictionaries.
did a search at http://thelongestlistofthelongeststuffatthelongestdomainnameatlonglast.com/