I learned today:

Pneumonoultramicroscopicsilicovolcanoconiosis (/ˌnjuːmənoʊˌʌltrəˌmaɪkrəˈskɒpɪkˌsɪlɪkoʊvɒlˌkeɪnoʊˌkoʊniˈoʊsɪs/ (listen)) is a word coined by the president of the National Puzzlers’ League as a synonym for the disease 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.

Etymology
This word was invented in 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”:

Pneumono­ultra­microscopic­silico­volcano­coniosis succeeded electrophotomicrographically as the longest word in the English language recognized by the National Puzzlers’ League at the opening session of the organization’s 103rd semi-annual meeting held yesterday at the Hotel New Yorker. The puzzlers explained that the forty-five-letter word is the synonym of a special form of pneumoconiosis caused by ultra-microscopic particles of silica volcanic dust…

Subsequently, the word was used in a puzzle book, Bedside Manna, after which time, members of the N.P.L. campaigned to include the word in major dictionaries.

This 45-letter word, referred to as “P45”, first appeared in the 1939 supplement to the Merriam-Webster New International Dictionary, Second Edition.