Pneumonoultramicroscopicsilicovolcanoconiosis is the longest word in any of the major English dictionaries. It is 45 letters in length and is a word that refers to a serious lung disease contracted from the inhalation of very fine silica particles, specifically from a volcano. This rare medical condition has no cure and is fatal.
The word was invented in 1935 by Everett Smith, president of the National Puzzlers League and was first published in the Merriam-Webster dictionary in 1939. It has held the distinction of being the longest word since that time. By the way, there really is a National Puzzlers League - http://www.puzzlers.org/dokuwiki/doku.php
If you try to pronounce it, you may run out of breath. Go ahead, give it a try:
NYOO-muh-noh-ul-truh-my-kruh-skop-ik-SIL-i-koh-vol-kay-noh-koh-nee-OH-siss.
Here is one author who used it in a sentence:
"This time the messages were longer and nastier. 'What on earth is the matter with you? Pneumonoultramicroscopicsilicovolcanoconiosis?'" Marion Thomas; Mystery Writer; Curriculum Corporation; 1998.
Okay, it may not be a useful word for most of us, but it is fun to know. For all of you Puzzlers out there, it may come up in a crossword sometime.
"In the course of my life, I have often had to eat my words, and I must confess that I have always found it a wholesome diet." Winston Churchill