Antidisestablishmentarianism is very occasionally found in genuine use, but it is most often cited as an example of a very long word. Other similar curiosities are

floccinaucinihilipilification

and

pneumonoultramicroscopicsilicovolcanoconiosis

(the longest word in this dictionary). The longest word to be encountered in Britain is the Welsh place name Llanfairpwllgwyngyllgogerychwyrndrobwllllantysiliogogogoch, which is generally abbreviated to Llanfair PG; this name was created in the 19th century