Definition of Apocalyptic

Here is a good definition of apocalyptic from one of the best scholarly studies on the topic:

"(ii) Apocalyptic seems essentially to be about the revelation of the divine mysteries through visions or some other form of immediate disclosure of heavenly truths

(iii) The use of the word apocalyptic to describe the literature of Judaism and early Christianity should, therefore, be confined to those works which purport to offer disclosures of the heavenly mysteries, whether as the result of vision, heavenly ascent or verbal revelations. Such a description also extends to those visionary reports which give evidence of the same kind of religious outlook as the apocalypses, even if the contexts in which they are now found cannot be said to conform to the literary genre of the apocalypse.

(iv) Although eschatology is an important component of the heavenly mysteries which are revealed in the apocalypses, it is difficult to justify the selection of this particular element as the basis of the definition of apocaalyptic. The consequence ofthis can lead to an indifference to the fact that apocalyptic is concerned with the revelation of a variety of different matters. Any attempt, therefore, to use the term apocalyptic as a synonym of eschatology must be rejected.....

(v) Although content and form should not in the first instance be the bases for a definition of apocalyptic, it cannot be denied that apocalyptic frequently finds expression in a particular literary genre. In Judaism this is usually an apocalypse granted to some great figure of Israel's past who then reveals to subsequent generations tge secrets which have been disclosed to him and gives advice to them about the sort of life which God expects of the righteous. "(Rowland, Christopher. "The Open Heaven: A Study of Apocalyptic in Judaism and Early Christianity", pp 90-72.)