Why is the ninth month called September?
late 14c., "revelation, disclosure," from Church Latin apocalypsis "revelation," from Greek apokalyptein "uncover, disclose, reveal," from apo- "from" (see apo-) + kalyptein "to cover, conceal" (see Calypso). The Christian end-of-the-world story is part of the revelation in John of Patmos' book "Apokalypsis" (a title rendered into English as "Apocalypse" c.1230 and "Revelations" by Wyclif c.1380).
Its general sense in Middle English was "insight, vision; hallucination;" meaning "a cataclysmic event" is modern. As agent nouns, apocalypst (1829), apocalypt (1834), and apocalyptist (1835) have been tried.
the Greek name of the Book of Revelation (q.v.).