No. it is not. The well-known song title from the movie Mary Poppins, Supercalifragilisticexpialidocious, with 34 letters, does appear in several dictionaries, but only as a proper noun, and defined in reference to the song title. Hence it may well be dismissed as not a "real" word.
The Guinness Book of Records, in its 1992 and subsequent editions, declared the "longest real word" in the English language to be floccinaucinihilipilification at 29 letters. Defined as the act of estimating as worthless, its usage has been recorded as far back as 1741. In recent times its usage has been recorded in the proceedings of the United States Senate by Senator Jesse Helms, and at the White House by Bill Clinton's press secretary Mike McCurry. It is the longest non-technical word in the first edition of the Oxford English Dictionary.
However, There are seemingly endless debates over what is the longest word in English, demonstrating that the idea of what constitutes a word is not as straightforward as it seems. Hyphenated or space-delimited compounds and proper nouns are linguistically considered words, but as they can grow with few limits, they are not counted.
Antidisestablishmentarianism (a 19th century movement in England opposed to the separation of church and state) at 28 letters is often popularly accepted as English's longest word, and is probably the best-known "longest word." Other versions such as Antidisestablishmentarianistically and Pseudoantidisestablishmentarianism are demonstrably longer, though, showing that 'popular acceptance' is not a guarantee of accuracy. (See also the "Constructions" section below.) "Antidisestablishmentarianism" is actually used seriously in academic and ecclesiastical writing about the Church of England when the concept arises, and although one may suspect that authors have gone out of their way not to avoid it, it does thus have a serious claim to be the longest real word in current English.
The word pneumonoultramicroscopicsilicovolcanoconiosis, also spelled pneumonoultramicroscopicsilicovolcanokoniosis, is defined as "a lung disease caused by the inhalation of very fine silica or quartz dust." At 45 letters, it is certainly the longest word ever to appear in a non-technical dictionary of English, the Oxford English Dictionary. However there are strong indications that the word was coined by puzzler Everett Smith in 1935 as a hypothetical long word that could result from the protraction of medical terms. The actual name of the disease is pneumoconiosis, which is 14 letters long.
A popular joke answer to the 'longest word' question is the word smiles, credited as the longest word because there is a mile between each s. Of course, by this reckoning the word beleaguered, which contains a league, is even longer.
According to some, the longest word is the word after the sentence "And now, a word from our sponsors".
Although only fourteen letters long, sesquipedalian deserves a mention. It is derived from a nonce word used by the Roman author Horace, in his work "Ars Poetica" (The Art of Poetry). The quote is as follows: "Proicit ampullas et sesquipedalia verba," which means, "He throws aside his paint pots and his words that are a foot and a half long". The Oxford English dictionary lists sesquipedalianism ("the practice of using words one and a half feet long"), and further derivations can be created as described in the "Constructions" section above.