As of 2015, the longest English word that is published in the Oxford Dictionaries is "pneumonoultramicroscopicsilicovolcanoconiosis" which is a term referring to a type of lung disease caused by silica dust. This word is 45 letters long.
Pneumonoultramicroscopicsilicovolcanoconiosis was created for the sake of being the longest English word in a dictionary; a more practical synonym for the word is "silicosis."
While some medical and scientific words are longer than pneumonoultramicroscopicsilicovolcanoconiosis, they are not recognized in the dictionary. The longest scientific word is 189,819 letters long and is the chemical name for a protein known as titin or connectin. This word takes over three hours to pronounce.