What is the longest word on dictionary?

The longest Word ON most dictionaries is probably "dictionary" (it might be a tie with "unabridged" or "collegiate").

The longest Word IN the dictionary ... well, it depends on the dictionary.

The longest Word that has something resembling a real meaning and isn't just an excuse for having a very long Word (or a technical medical or scientific term) is probably antidisestablishmentarianism (there was at one time... there may still be, I don't know.. a movement seeking to get rid of, or "disestablish", the Church of England; the Word in question describes a movement opposing that movement, hence the "anti"). There's a supposed Word that means "the act of estimating as worthless" that's something like floccinaucinihilipilification, but it's only a Word in the most technical sense; no one ever uses it except to try to win "longest Word" contests.

Also, there are any number of long medical terms. One example: Pneumonoultramicroscopic-silicovolcanoconiOSis (A type of disease)

In addition, the way that chemical names are formed means the IUPAC name of any protein is an extremely long and largely meaningless (to non-biochemists) "Word," but you wouldn't find any of them in most dictionaries.