What is the longest word in your dictionary?
The longest word in the Infoplease Dictionary is pneumonoultramicroscopicsilicovolcanoconiosis
In a field of over 122,000 headwords, pneumonoultramicroscopicsilicovolcanoconiosis beats out its closest competitor (supercalifragilisticexpialidocious) by 11 letters. Both have objections to them; the former was deliberately invented to be the longest word in the language, and has never been used in any other context, while the latter is an invented nonsense word. Other, more legitimate, long words include floccinaucinihilipilification and antidisestablishmentarianism.
One might also count trinitrophenylmethylnitramine, and any number of chemical compounds whose names could go on for pages if anybody cared to describe them that way.
—The Fact Monster