What is the longest word known to the english dictionary?

It depends on the dictionary, and the criteria you apply to what kinds of Words are considered to qualify. The spelling out of chemical names, for example, can result in Words of truly eye-watering length (titin, a protein, runs to 189,819 letters!), but are rarely counted among "long Words" in the English language.

This is a popular question, however, and consensus has established some definite front-runners in the Longest Word in the English Language (by letter count, which is a dispute in itself) category.

• pneumonoultramicroscopicsilicovolcanoconiOSis, which is included in the Oxford English Dictionary, is 45 letters long. It's a technical term for a lung disease caused by breathing in ultramicroscopic volcanic particles.
• supercalifragilisticexpialidocious, a nonsense Word famous from the Mary Poppins film, is 34 letters long. Many dictionaries include it, though it was "coined"-that is to say, made up.
• floccinaucinihilipilification, another coined term, is 29 letters long. It was devised from a mashed-together and purposefully redundant bunch of Latin stems and is usually defined roughly as "the act of regarding or describing something as worthless."
• antidisestablishmentarianism, at 28 letters, is often cited as the longest non-coined, non-technical Word in the English language.

In one of his comedies, Aristophanes coined a Word as the name of a fictitious foodstuff that was 171 letters long in Greek, and 183 letters long when transliterated into English. The longest Word to appear in Shakespeare's works (honorificabilitudinitatibus) is 27 letters long.