The longest word in any major English language dictionary is pneumono­ultra­micro­scopic­silico­volcano­coniosis, a 45-letter word supposed to refer to a lung disease contracted from the inhalation of volcanic ash, but research has discovered that this word was originally intended as a hoax. It has since been used in a close approximation of its originally intended meaning, lending at least some degree of validity to its claim.[1]

The Oxford English Dictionary contains pseudopseudohypoparathyroidism (30 letters).

The longest non-technical word is flocci­nauci­nihili­pili­fication at 29 letters. Consisting of a series of Latin words meaning "nothing" and defined as "the act of estimating something as worthless," its usage has been recorded as far back as 1741.[2][3][4] In recent times its usage has been recorded in the proceedings of the United States Senate by Senator Robert Byrd [5], and at the White House by Bill Clinton's press secretary Mike McCurry, albeit sarcastically.[6]

Anti­dis­establishment­arianism (a nineteenth century movement in England opposed to the separation of church and state) at 28 letters is still in colloquial currency for being one of the longest words in the English language.

The longest word which appears in William Shakespeare's works is the 27-letter honorific­abilitud­initatibus, appearing in Love's Labour's Lost. This is arguably an English word (rather than Latin), but only because it was Shakespeare who used it.

The humuhumu-nukunuku-a-pua‘a, or reef triggerfish, is Hawaii's official state fish.[7] At 22 letters (including the okina) it is one of the best known very long one-word names for an animal. It is occasionally quipped that the name is longer than the fish.

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 below.

I knew about this word, but I don't really like how this word is so famous.... I mean sure it's long, but it's not like anyone has ever used it before :S

HEPATICO­CHOLANGIO­CHOLECYST­ENTERO­STOMIES = a surgical creation of a connection between the gall bladder and a hepatic duct and between the intestine and the gall bladder.
This is the longest word in Gould's Medical Dictionary.

SUPER­CALI­FRAGI­LISTIC­EXPI­ALI­DOCIOUS = song title from the Walt Disney movie Mary Poppins.
It is in the Oxford English Dictionary.

HIPPOPOTO­MONSTRO­SESQUIPED­AL­IAN = pertaining to a very long word.
From Mrs. Byrne's Dictionary of Unusual, Obscure and Preposterous Words.

Wow, i have got to say that whoever makes words longer than 10 letters should be hit repeatedly. Hard. who is the idiot who would come up with such stupidity???

oh well, the world is full of idiots...nothing we do or say will change that.

this word was originally intended as a hoax.