I saw a very long thread about this topic, and I wanted to set the record completely straight. You will notice that I mentioned ENGLISH LANGUAGE in the subject. Unless someone can give us a reliable definition for the extremely long finnish word I believe we should refrain from discussing it.
The main debate appears to be whether PNEUMONOULTRAMICROSCOPICSILICOVOLCANOCONIOSIS is a legitimate word, as it was coined simply to make a new longest word but has a real definition. Many have said that FLOCCINAUCINIHILIPILIFICATION is the longest REAL word, because of previously stated complication with PNEU..., which is described as "factitious", meaning artificial. However, it appears in four respectable dictionaries and as I have said, has a real definition. Because it is "real" enough to be actually used in conversation (although maybe only at medical colleges) I believe it has enough status to be considered a real word, and thus, the longest word in the english language.
Chemical names should be thrown out of the question. Seeing as they are basically combinations of many different amino acids, proteins, enzymes and the like, they are not really words but more letter combinations.
Antidisestablishmentarianism is a word commonly cited as an example of the longest english word. However, it is shorter than FLOCCI... or PNEUM... which obviously makes it NOT the longest.
Some say smiles is the longest because there is a mile between each S. Also, Someone suggested beleaguered, because a league is longer than a mile. Both are true and interesting interpretations (thank you very much to all of those who suggested these), but the thread was asking for longest by letter count, so neither of these count (yes, I may be too thorough).
A very interesting link to a page about many of the vowel, syllable, etc. debate, as I do not want to bother with them, is: http://www.fun-with-words.com/word_vowels.html
So there you have it. I hope this is definitive as to the longest word in the english language. Feel free to post back, but please give me a sensible response, and this discussion is NOT about place names, foreign words, or chemical names, so do not use those.
Thank you