The longest phrase within the English language is pneumonoultramicroscopicsilicovolcanoconiosis, a lung illness attributable to the inhalation of very nice silica mud, normally present in volcanoes.
The phrase was first coined by American doctor and writer Everett M. Smith in 1935. He created the phrase from the Greek roots pneumono (lung), extremely (very), microscopic (small), silico (silicon), volcano (volcano), and -osis (situation). The phrase is 45 letters lengthy and is pronounced as noo-moh-no-ul-tra-mih-kroh-skoh-pih-ko-sih-lih-koh-voh-lka-noh-koh-nee-oh-sis.