The longest phrase within the English language is pneumonoultramicroscopicsilicovolcanoconiosis, a lung illness brought on by the inhalation of very nice silica mud, often present in volcanoes.
The phrase was first coined by American doctor and creator 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.