What is the longest English word?
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 Llanfairpwllgwyngyllgogerychwyrndrobwllllantysiliogogogoch. 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