What is the longest word in the English language?

In major English language dictionaries, the longest word is "pneumonoultramicroscopicsilicovolcanoconiosis" (45 characters).

This word designates a lung disease more commonly known as "silicosis".

The word "unconstitutionally" (25 characters) - until now considered to be the longest word in the French language - has recently been dethroned by the word "intergouvernementalisations" (27 characters).