How many letters are there in the longest word in the English language published in a dictionary?

Pneumonoultramicroscopicsilicovolcanoconiosis is an English word that refers to a lung disease that is otherwise known as silicosis. It is the longest word in the English language published in a dictionary, the Oxford English Dictionary, which defines it as "an artificial long word said to mean a lung disease caused by inhaling very fine ash and sand dust."

Silicosis is a form of occupational lung disease caused by inhalation of crystalline silica dust, and is marked by inflammation and scarring in the form of nodular lesions in the upper lobes of the lungs. It is a type of pneumoconiosis.

This word was invented at the annual meeting of the National Puzzlers' League (N.P.L.) by its president Everett M. Smith. The word featured in the headline for an article published by the New York Herald Tribune on February 23, 1935, titled "Puzzlers Open 103rd Session Here by Recognizing 45-Letter Word":