What is the longest word in English?

smiles - it it one mile between two S's 

there are other longer, but this one is the only one that makes part of english dictionaires "pneumonoultramicroscopicsilicovolcanoconiosis"

pneumonoultramicroscopicsilicovolcanoconiosis

The longest word in the English language  is "Pneumonoultramicroscopicsilicovolcanoconiosis," a word that refers to a lung disease contracted from the inhalation of very fine silica particles, specifically from a volcano; medically, it is the same as silicosis.

I used to think that "pneumonoultramicroscopicsilicovolcanoconiosis" is the longest word in English, but I have found even a longer word. It is the chemical name for "titin" , and it would be ridiculous to copy and paste the  word which has more than one hundred thousand letters, so I will just  paste a small part of it:

Methionylthreonylthreonylglutaminylarginyltyrosylglutamylserylleucylphenylalanylalanylglutaminylleuc yllysylglutamylarginyllysylglutamylglycylalanylphenylalanylvalylprolylphenylalanylvalylthreonylleucylgl ycylaspartylprolylglycylisoleucylglutamylglutaminylserylleucyllysylisoleucylaspartylthreonylleucylisoleu cylglutamylalanylglycylalanylaspartylalanylleucylglutamylleucylglycylisoleucylprolylphenylalanylseryla spartylprolylleucylalanylaspartylglycylprolylthreonylisoleucylglutaminylasparaginylalanylthreonylleucyl ...

Pedro Brites is right.

I thought of

antidisestablishmentarianism,

but that is shorter than the word given by Pedro.