Pneumonoultramicroscopicsilicovolcanoconiosis*
It doesn't matter if you can spell the word. Do you actually know what the words mean?
Antidistestablishmentarianism used to by my longest word until I learned the above one. It is a lung disease, by the way.
Llanfairpwllgwyngyllgogerychwyrndrobwllllantysiliogogogoch *Llanfairpwllgwyngyllgogerychwyrndrobwllllantyssiliogogogoch
Does it count if it is the name of a village and not a definable word?