longest English word

pneumonoultramicroscopicsilicovolcanoconiosis

NOO-muh-noh-UL-truh-MY-kruh-SKOP-ik-SIL-i-koh-VOL-kay-no-koh-NEE-o-sis)

A lung disease caused by inhaling fine particles of silica.

At 45 letters, it's the longest word in any English language dictionary.
It's a trophy word: its only job is to serve as the longest word. In day-to-day use, its nine-letter synonyms "silicosis" or "black lung" work just as well, and the latter is more descriptive. Whatever you call it, it is deadly.