c. 1300, revelacioun, "disclosure of information or knowledge to man by a divine or supernatural agency," from Old French revelacion and directly from Latin revelationem (nominative revelatio), noun of action from past-participle stem of revelare "unveil, uncover, lay bare" (see reveal).
The general meaning "disclosure of facts to those previously unaware of them" is attested from late 14c.; meaning "striking disclosure" is from 1862. As the name of the last book of the New Testament (Revelation of St. John), it is attested from late 14c. (see apocalypse); as simply Revelations, it is recorded by 1690s.
late 14c., "revelation, disclosure," from Church Latin apocalypsis "revelation," from Greek apokalyptein "uncover, disclose, reveal," from apo "off, away from" (see apo-) + kalyptein "to cover, conceal," from PIE root *kel- (1) "to cover, conceal, save." The Christian end-of-the-world story is part of the revelation in John of Patmos' book "Apokalypsis" (a title rendered into English as pocalipsis c. 1050, "Apocalypse" c. 1230, and "Revelation" by Wyclif c. 1380).
Its general sense in Middle English was "insight, vision; hallucination." The meaning "a cataclysmic event" is modern (not in OED 2nd ed., 1989); apocalypticism "belief in an imminent end of the present world" is from 1858. As agent nouns, "author or interpreter of the 'Apocalypse,'" apocalypst (1829), apocalypt (1834), and apocalyptist (1824) have been tried.
late 14c., used in Revelation ix.11 of "the angel of the bottomless pit," and by Milton of the pit itself, from Hebrew Abhaddon, literally "destruction," from abhadh "he perished." The Greek form was Apollyon.
"cataclysmic final conflict," 1811, figurative use of the place-name in Revelation xvi.16, site of the great and final conflict, from Hebrew Har Megiddon "Mount of Megiddo," a city in central Palestine, site of important Israelite battles.
semi-precious stone, a cloudy white variety of quartz, c. 1300, from Latin calcedonius, a Vulgate rendering of Greek khalkedon in Revelation xxi.19; found nowhere else. "The word is of very complicated history" [OED]. Connection with Chalcedon in Asia Minor "is very doubtful" [OED].