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.
The seventh longest word in the dictionary is "undistinguishableness."
The longest Tagalog word in the dictionary is "pambansangmaakiabot," which translates to "national scope."
The longest word in the Oxford dictionary that begins with 'f' is floccipaucinihilipilification. -29 letters.
The longest word in the English dictionary is Pneumonoultramicroscopicsilicovolcanoconiosis. This is a lung disease.
No, it is not. Pneumonoultramicroscopicsilicovolcanoconiosis is.
philosophy
Is, and/or In
Pneumonoultramicroscopicsilicovolcanoconiosis
the longest word in the dictionary
original APEX =)
Supercalifragilisticexpialidocious
Mississippi