The longest word for the name of something in English is 85 letters long and is the name of a hill in New Zealand,
As this is the name of a place, it does not truly count, so the actual longest word in a major dictionary is a word which was in fact coined to be the longest word in English,
However, in a comedy by Aritophanes (the Assemblywomen), the author coins the word
which is the name of a fictional entree composed of fish, fowl, sauces and all kinds of dainties, and of more particular interest to us, is 183 letters long.
However, all of this comes into conflict, as some people think the real longest word in the English language is the scientific name for the protein connectin, which I am not going to put down as it has 189,189 letters, effectively dwarfing any possible competition.