antidisestablishmentarianism

pneumonoultramicroscopicsilicovolcanoconiosis "a lung disease caused by breathing in very small pieces of ash or dust" https://dictionary.cambridge.org/fr/dictionnaire/anglais/pneumonoultramicroscopicsilicovolcanoconiosis

It isn't a very interesting question because there's no objective way to decide what is a real "word." Dictionaries try to record words that are in real use--not just one example by one writer. They try to record words that belong to the general vocabulary, not to some specialized technical vocabulary. These are arbitrary, based on the judgement of the people who compile the dictionary. The real, official IUPAC name for a protein called Titin is 189,819 letters long, and it can be argued that it is an English word. Or not. The standard answers refer to the longest word in the Merriam-Webster Third New International Dictionary, Unabridged, and the (full) Oxford English Dictionary. During the period in the 1800s when there a political debate about the Established Church in England, words like "establishmentarian," "disestablishment," etc. were really used. A Google Books search turns up examples like (1868) "The Low school is Establishmentarian to a man, " and (1902) "Staunch Establishmentarian as he was, he was no Erastian," and (1906) "He was anti-ochlocratic, not, I think, essentially anti-democratic. He was anti-disestablishmentarian, and he was anti-innovationist—that is, he was opposed to innovation for innovation's sake..." So words on this pattern were really used, although I seriously doubt that the longer combinations were common. I hate to say this, but the invented word "supercalifragilisticexpialidocious" from the Disney movie, "Mary Poppins," is probably sung and spoken aloud more often than any of the famous "longest words in the dictionary."