The longest words in Oxford Dictionaries Online are:

• antidisestablishmentarianism opposition to the disestablishment of the Church of England - 28 letters
• floccinaucinihilipilification the estimation of something as worthless - 29 letters
• pneumonoultramicroscopicsilicovolcanoconiosis a supposed lung disease - 45 letters

You're unlikely to come across these words in genuine use: they're generally just provided as answers to questions about the longest words in the English language. In terms of sheer size, however, the longest word to be found in Britain is the Welsh place name Llanfairpwllgwyngyllgogerychwyrndrobwllllantysilio gogogoch. For obvious reasons, it's usually abbreviated to Llanfair PG.

The 20-volume historical Oxford English Dictionary includes other very long words, most of which are highly technical. These include:

• otorhinolaryngological - 22 letters
• immunoelectrophoretically - 25 letters
• psychophysicotherapeutics - 25 letters
• thyroparathyroidectomized - 25 letters
• pneumoencephalographically - 26 letters
• radioimmunoelectrophoresis - 26 letters
• psychoneuroendocrinological - 27 letters
• hepaticocholangiogastrostomy - 28 letters
• spectrophotofluorometrically - 28 letters
• pseudopseudohypoparathyroidism - 30 letters

People sometimes ask whether a DNA string can be considered as the longest English word, given that they can run to many thousands of letters. The answer is no: they're regarded as chemical names rather than genuine words in the sense of meaningful items of vocabulary. The same is true of the formal names of chemical compounds. These can be almost unlimited in length (for example, aminoheptafluorocyclotetraphosphonitrile, 40 letters) and many contain numerals, Roman and Greek letters, and other symbols, as well as ordinary letters. We don't tend to regard these terms as proper 'words'.

المشاركة الأصلية كتبت بواسطة Ahmed456 (المشاركة 2486600) smiles بين الحرف الاول والحرف الاخير ميلا كاملا .. ههههههههههههههه

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Ornicopytheobiblopsychocrystarroscioaerogenethliom eteoroaustrohieroanthropoicichthyopyrosiderochpnom yoalectryoophiobotanopegohydrorhabdocrithoaleuroal phitohalomolybdoclerobeloaxinocoscinodactyliogeoli thopessopscphocatoptrotephraoeirochiroomychodactyl oarithstichooxogeloscogastrogyrocerobletonooenosca pulinaniac (310 letters)
''A deluded human who practises divination or forecasting''

Lopadotemachoselachogaleokranioleipsanodrimhypotri mmatosilphioparaomelitokatakechymenokichlepi***syp hophattoperisteralektryonoptekephalliokigklopeleio lagoiosiraiobaphetraganopterygon(182 letters)
''The name of a 17-ingredient Greek dish''

Bababadalgharaghtakamminarronnkonnbronntonnerronnt uonnthunntrovarrhounawnskawntoohoohoordenenthurnuk (100 letters)
An arsenic-containing drug

Twoallbeefpattiesspecialsaucelettucecheesepickleso nionsonasesameseedbun (71 letters)
Made of several words was used in 1975 as advertising jingle by McDonald’s Restaurant to describe the ‘Big Mac’ sandwich.

Aequeosalinocalcalinoceraceoaluminosocupreovitriol ic (52 letters)
''Spa waters at bath, England''

Osseocarnisanguineoviscericartilaginonervomedullar y (51 letters)
''Having the structure of the human body'' / ''of bone, flesh, blood, organs, gristle, nerve, and marrow.''

Pneumonoultramicroscopicsilicovolcanokoniosis (45 letters)
''A lung disease caused by the inhalation of very fine silica dust, causing inflammation in the lungs ''

Osteosarchaematosplanchnochondroneuromuelous (44 letters)
Adj.

Hepaticocholangiocholecystenterostomies (39 letters)
''A surgical creation of a connection between the gall bladder and a hepatic duct and between the intestine and the gall bladder.'' (Gall bladder surgery)

Tetramethyldiaminobenzhydrylphosphinous (39 letters)
''It is a type of acid''

Dimethylamidophenyldimethylpyrazolone (37 letters)
Chemical term

Formaldehydetetramethylamidofluorimum (37 letters)
Chemical term

Praetertranssubstantiationalistically (37 letters)
Adj. The act of surpassing the act of transubstantiation, which refers specifically to the transformation of the bread and wine into the body and blood of Christ during the Roman Catholic mass. / The bread and wine got transformation in the body and blood of the Christ.

Hippopotomonstrosesquippedaliophobia (36 letters)
The fear of long words

Supercalifragilisticexpialidocious (34 letters)
Adj. Fantastic, very wonderful, Song title from the Walt Disney movie Mary Poppins.

Pseudoantidisestablishmentarianism (34 letters)
"False opposition to the withdrawal of State support from a church" or "Extreme opposition to the withdrawal of State supported church" or ''False opposition to the separation of churchand ofthe state.''

Hippopotomonstrosesquipedalianism (33 letters)
Thelove oflong words

Dichlorodiphenyltrichloroethane (31 letters)
DDT is an organochlorineinsecticide which is a white, crystalline solid, tasteless and almost odourless

Hippopotomonstrosesquipedalian (30 letters)
Adj. Of or pertaining to extremely long words

Pseudopseudohypoparathyroidism (30 letters)
A mild inherited condition in which the sufferer has symptoms similar to those of pseudohypoparathyroidism

Floccinaucinihilipilification (29 letters)
The act or habit of describing or regarding something as unimportant, of having no value or being worthless / The estimation of something as worthless

Hexa***ioihexekontahexaphobia (29 letters)
Fear of the number 666

Trinitrophenylmethylnitramine (29 letters)
Sensitive explosive compound used to make detonators and explosive booster charges ( abbrev. Tetryl)(C7H5N5O8)

Hepaticocholangiogastrostomy (28 letters)
An atrophy of the body or part of the body, especially pulmonary tuberculosis

Spectrophotofluorometrically (28 letters)
Adv. 'using a spectrophotofluorometer' [23 letters]

Antitransubstantiationalist (27 letters)
One who disagrees with the concept of or belief in the transubstantiation doctrine of the Catholic Church/ one who doubts the validity of transubstantiation / one who doubts that consecrated bread and wine actually change into the body and blood of Christ.

Electroencephalographically (27 letters)
Adv. Related to the recording of electrical activity along the scalp. EEG measures voltage fluctuations resulting from ionic current flows within the neurons of the brain. / detecting and recording brain waves(n. Electroencephalography [22 letters], EEG)

Ethylenediaminetetraacetate (27 letters)
Salt of EDTA

Psychoneuroendocrinological (27 letters)
Adj. Of or pertaining to psychoneuroendocrinology [24 letters].

Ethylenediaminetetraacetic (26 letters)
A polyamino carboxylic acid and a colourless, water-soluble solid. (EDTA)

Pneumoencephalographically (26 letters)
Adj. related to a medical procedure in which most of the cerebrospinal fluid is drained from around the brain and replaced with air, oxygen, or helium to allow the structure of the brain to show up more clearly on an X-ray image. (n. Pneumoencephalography [21 letters])

Radioimmunoelectrophoresis (26 letters)
Immunoelectrophoresis in which the substances separated in the electrophoretic system are identified by radioactive labels on antigens or antibodies (immunoelectrophoresis using antigens or antibodies labelled with radioisotopes)

Immunoelectrophoretically (25 letters)
A way to find out how much of a certain immunoglobulin is in someone's blood.

Psychophysicotherapeutics (25 letters)
Remedial treatment of mind and body.

Thyroparathyroidectomized (25 letters)
Adj. Related to excision of both the thyroid and parathyroid glands (n. thyroparathyroidectomy [22 letters])

Friggatriskaidekaphobia (24 letters)
The fear of Friday the 13th

Pseudohypoparathyroidism(24 letters)
A condition caused by resistance to the parathyroidhormone

Paraskevidekatriaphobia (23 letters)
The fear of Friday the 13th

Otorhinolaryngological (22 letters)
Adj. Related to the medical specialty concerned with diseases of the ear, nose, and throat (n.otorhinolaryngology [19 letters])

Honorificabilitudinity (22 letters)
The quality of being honourable

Disproportionableness (21 letters)
The state or condition of being disproportionable.

Incomprehensibilities (21 letters)
The quality of being incomprehensible

Triskaidekaphobia (17 letters)
''Fear of the number 13''

Taumatawhakatangihangakoauauotamateaturipukakapiki maungahoronukupokaiwhenuakitanatahu (85 letters) ……………..A hill in New Zealand.
Gorsafawddachaidraigodanheddogleddolonpenrhynareur draethceredigion (66 letters)……. A town in Wales.
Llanfairpwllgwyngyllgogerychwyrndrobwllllantysilio gogogoch (58 letters)……….. A town in North Wales.
Chargogagogmanchargogagogcharbunagungamog (41 letters)………. Another name for Lake Webster in Massachusetts, USA.
Chargoggagoggmanchauggagoggchaubunagungamogg (44 letters)
Chargoggagoggmanchauggagoggchabunagungamaugg (44 letters)
Chargoggagoggmanchauggagoggchaubunagungamaugg (45 letters)
Nunathloogagamiutbingoi (23 letters)………… The Eskimo name for some dunes in Alaska.

§ http://www.english-for-students.com/Longest-Words.html
§ http://www.fun-with-words.com/word_longest.html
§ http://www.a-z-dictionaries.com/blog/a-collection-of-word-oddities-and-trivia-page-11/
§ http://www.almaany.com/home
§ http://sites.google.com/site/parthochoudhury/longwords
§ http://jeff560.tripod.com/words11.html
§ http://longestword.nigeltomm.com/
http://starprofm.blogspot.com/2012/03/blog-post.html