Technical terms
A number of scientific naming schemes can be used to generate arbitrarily long words.
Gammaracanthuskytodermogammarus loricatobaicalensis is sometimes cited as the longest binomial name—it is a kind of amphipod. However, this name, proposed by B. Dybowski, was invalidated by the International Code of Zoological Nomenclature.
Aequeosalinocalcalinoceraceoaluminosocupreovitriolic, describing the spa waters at Bath, England, is attributed to Dr Edward Strother (1675-1737). The word is composed of the following elements:
Aequeo: equal (Latin, aequo)
Salino: containing salt (Latin, salīnus)
Calcalino: calcium (Latin, calx)
Ceraceo: "waxy" (Latin, cera)
Aluminoso: alumina (Latin)
Cupreo: from Copper
Vitriolic: containing vitriol (in this case Copper sulphate pentahydrate)
John Horton Conway and Landon Curt Noll developed an open-ended system for naming powers of 10, in which one sexmilliaquingentsexagintillion, coming from the Latin name for 6560, is the name for 103(6560+1) = 1019683. In British usage, it would be 106(6560) = 1039360.
Names of chemical compounds can be extremely long if written as one word, which is sometimes done. An example of this is sodiummetadiaminoparadioxyarsenobenzoemethylenesulphoxylate, an arsenic-containing drug. There are also other chemical naming systems, using numbers instead of "meta", "para" etc. as descriptive dividers, breaking up the name, which then no longer can be considered a single long word.
The IUPAC nomenclature for organic chemical compounds is open-ended, giving rise to such words as Methionylthreonylthreonyl...isoleucine (189,819 letters).
British science writer Brian J. Ford published the most polysyllabic book title (popularly known, for short, simply as Nonscience) in 1971. It is a satire on those who delude the public with unnecessarily complex lengthy terms.
Place names
The longest place name in the world spelled in English is Krungthepmahanakornamornratanakosinmahintarayutthayamahadilokphopnopparatrajathaniburiromudomrajaniwesmahasatharnamornphimarnavatarnsathitsakkattiyavisanukamprasit in Thailand, although it is more commonly known as Bangkok.
There is some debate as to whether a place name is a legitimate word. However, the longest officially recognized place name in an English-speaking country is Taumatawhakatangihangakoauauotamateapokaiwhenuakitanatahu (85 letters) which is a hill in New Zealand.
The longest place name in the United States (45 letters) is Chargoggagoggmanchauggagoggchaubunagungamaugg, a lake in Webster, Massachusetts. This is incorrectly said to be Algonquin for "you fish your side of the water, I fish my side of the water, nobody fishes the middle". It actually means "Englishmen at Manchaug at the Fishing Place at the Boundary". The lake is known to Americans as Webster Lake. The longest hyphenated names in the U.S. are Winchester-on-the-Severn, a town in Maryland, and Washington-on-the-Brazos, a notable place in Texas history.
The 58 character name Llanfairpwllgwyngyllgogerychwyrndrobwllllantysiliogogogoch is the famous name of a town in Anglesey in the United Kingdom. This place's name is actually 51 letters long, as certain character groups in Welsh are considered as one letter, for instance ll, ng and ch. It is generally agreed, however, that this invented name, adopted in the mid-19th century, was contrived solely to be the longest name of any town in Britain. The official name of the place is Llanfairpwllgwyngyll, commonly abbreviated to "Llanfairpwll" or the somewhat jocular "Llanfair PG". The longest station name in the UK, at 68 letters, is also in Wales: Gorsafawddacha'idraigodanheddogleddollônpenrhynareurdraethceredigion was specifically contrived to "beat" Llanfairpwllgwyngyll.
In Ireland, the longest English placename at 22 letters is Muckanaghederdauhaulia (from the Irish language, Muiceanach Idir Dhá Sháile, meaning pig-marsh between two saltwater inlets) in County Galway. If this is disallowed for being derived from Irish, or not a town, the longest at 19 letters is Newtownmountkennedy in County Wicklow.
It is questionable whether any of the above (with the exception of Newtownmountkennedy) are properly considered English words, being derived from Māori, Nipmuck, Welsh and Irish words respectively, or being a conjunction of individual English words.
Scrabble
The longest technically legal Scrabble word in North American play is ethylenediaminetetraacetates (28 letters). It is the plural of a word found in Merriam-Webster's Collegiate Dictionary, 10th edition, which was the dictionary of reference in North American Scrabble play for base words of at least 10 letters, and their inflections of at least 10 letters, until June 16, 2003. Naturally, this 15-letter plus word is 'legal' in name only, since it can never be used in a game for the very simple reason that it would not fit on the board. There are many 15-letter words; the highest-scoring word on a Scrabble board is oxyphenbutazone (1458 points) when played across a set of three triple word scores. Arguably, however, higher scoring at 1674 points is sesquioxidizing (which scores 62 in its own right, but considerably higher when tripled three times - and possibly using other premium squares). This word does not exist in its own right in places like Webster's dictionary - however the roots of the word do see e.g. sesquioxide and oxidizing. For words that exist in other dictionaries, benzoxycamphors scores 59 before inflation.
Words with certain characteristics of notable length
The longest word in the English language containing only one vowel is strengths, while scraunched is the longest monosyllabic word in current usage. Twyndyllyngs is the longest word without any of the common vowel letters a, e, i, o, or u (although y functions as a vowel in this word). Euouae, at six letters long, is the longest English word consisting only of vowels, and the word with the most consecutive vowels. It is a medieval musical term. However, u was often used interchangeably with v, and the variant "evovae" is occasionally used.
The longest words with no repeated letters are dermatoglyphics, misconjugatedly and uncopyrightable.
The longest word whose letters are in alphabetical order is the eight-letter Aegilops, a grass genus. The seven-letter addeems (from the archaic verb addeem, meaning to award), alloquy (an archaic or literary word for an address), beefily (in a beefy manner), billowy (like a wave or surge), dikkops (a South African bird) and gimmors (plural of gimmor, an old-fashioned word for a mechanical contrivance) are also close.
The longest word typable with only the left hand (using conventional hand placement on a QWERTY keyboard) is tesseradecades or the more common but sometimes hyphenated sweaterdresses. Conversely, using the right hand alone, the longest word that can be typed is johnny-jump-up, or, excluding hyphens, hypolimnion. The longest word typable using only the top row of letters is not typewriter, as is commonly believed: teetertotter is longer, though sometimes hyphenated. The longest words typable by alternating left and right hands are antiskepticism and leucocytozoans respectively.
The longest word with the vowels in order is abstemiously, although arseniously and facetiously are close.
Jokes
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.
A popular playground joke in Britain states that the longest word in the English language is elastic because it will always stretch.
American comedian Red Skelton claimed that the longest word is the one following the announcement, 「And now a word from our sponsor」!
Also, when asked "Which two words have the most letters?" the answer can be given as " Post Office".
