The longest word in Webster's Dictionary is "pneumonoultramicroscopicsilicovolcanoconiosis," as of 2015. The word contains 45 letters and refers to a disease that develops in the lungs from inhaling very fine irritants. Other exceptionally long words in Webster's Dictionary are "antidisestablishmentarianism" and "floccinaucinihilipilification" which are 28 and 29 letters long, respectively.
The existence of "pneumonoultramicroscopicsilicovolcanoconiosis" debunks a common myth that the longest word in the English language is "antidisestablishmentarianism." In fact, the former is longer than the latter by 17 letters, with the latter being the third longest word in the English language. Most exceptionally long words have become obsolete in a practical sense, as they can be excessive and confusing and, in most cases, shorter options exist. They are now mostly used as fodder when discussing long words and the meanings are largely irrelevant.