Apocalypse means unveiling or revelation, and the drift of the English word toward "catastrophe" reverses the emphasis of the literature it names. In technical usage an apocalypse is a genre: a narrative revelation, given to a human recipient by an otherworldly mediator, disclosing a reality that is both temporal, a coming judgement and deliverance, and spatial, involving another world. That definition was formulated by a Society of Biblical Literature working group under John J. Collins in 1979 and is standard in the field.
The genre flourished in Judaism from roughly the third century BCE to the second century CE: 1 Enoch, Daniel 7-12, 4 Ezra, 2 Baruch, the Apocalypse of Abraham, and a body of material among the Dead Sea Scrolls. Its recurring features are pseudonymity, the attribution of the work to an ancient worthy; vision followed by angelic interpretation; the division of history into set periods; symbolic beasts and numbers; and a resolution that arrives from outside history rather than developing within it. The setting is characteristically one of crisis, real or perceived, and the final form of Daniel is generally dated to the persecution under Antiochus IV in about 167-164 BCE.
Christianity took the genre up in Revelation, in the synoptic apocalypse of Mark 13, and in parts of Paul, and the New Testament book gave the word its currency. Revelation's interpretation has divided readers since antiquity among those who read it as addressing the writer's own time, as mapping the whole course of church history, as describing the end of the age, and as symbolic representation of realities recurring in every age; each of these has serious defenders and the choice among them largely determines what the book is taken to say. Islamic tradition developed its own literature of the signs of the Hour, largely in hadith rather than in the Qur'an, and Zoroastrian apocalyptic in the Bahman Yasht is among the oldest material of the type.
Two things about apocalyptic are worth holding apart. Historically, it is characteristically the literature of the powerless: it does not propose a programme, it counsels endurance, and its consolation is that the outcome has already been decided elsewhere. Sociologically, apocalyptic expectation has nevertheless repeatedly produced movements that act, the Jewish revolts, medieval millenarian risings, the Anabaptist kingdom at Munster in 1534-35, the Taiping in the nineteenth century, and, in a small number of modern cases, groups convinced the end was imminent have turned to violence. Scholars of new religious movements consistently caution against reading that pattern back onto apocalyptic belief generally, which is held peaceably by very large numbers of people and functions for most of them as a way of enduring the present rather than a licence to hasten its end.