World's Longest Word

What is the longest English word?

There is World's most longest words along length of long word. 

World's longest word is " Methionylthreonylthreonylglutaminylarginyl.....isoleucine " length of this word is  (189,819) letters, and its a chemical term.

The identity of the longest word in English depends upon the definition of what constitutes a word in the English language, as well as how length should be compared. In addition to words derived naturally from the language's roots (without any known intentional invention), English allows new words to be formed by coinage and construction; place names may be considered words; technical terms may be arbitrarily long. Length may be understood in terms of orthography and number of written letters, or (less commonly) phonology and the number of phonemes. Reference (Wikipedia)

• Methionylthreonylthreonylglutaminylarginyl.....isoleucine -  Letters (189,819)
• category (Chemical name of titin)

• Pneumonoultramicroscopicsilicovolcanoconiosis - a supposed lung disease - 45 letters
  

There are some more longest word in  20-volume historical Oxford English Dictionary  most of which are highly technical terms. These include:

• Otorhinolaryngological - 22 letters
• Psychophysicotherapeutics - 25 letters
• Immunoelectrophoretically - 25 letters
• Thyroparathyroidectomized - 25 letters
• Radioimmunoelectrophoresis - 26 letters
• Pneumoencephalographically  - 26 letters
• Psychoneuroendocrinological - 27 letters
• Spectrophotofluorometrically - 28 letters
• Hepaticocholangiogastrostomy - 28 letters
• Pseudopseudohypoparathyroidism - 30 letters