The Collaborative International Dictionary of English v.0.48:

Apocalyptic \A*poc`a*lyp"tic\, Apocalyptical
\A*poc`a*lyp"tic*al\, a. [Gr. ?.]
   1. Of or pertaining to a revelation, or, specifically, to the
      Revelation of St. John; containing, or of the nature of, a
      prophetic revelation.
      [1913 Webster]

   2. pertaining to, characteristic of, or resembling an
      apocalypse, in senses 3 or 4.
      [1913 Webster]

   Apocalyptic number, the number 666, mentioned in --Rev.
      xiii. 18., in which it is described as the number of the
      "beast of the earth". It has been variously interpreted.
      Some fundamentalist Christians consider it to be the
      number of the Devil, and avoid or fear objects containing
      that number.
      [1913 Webster +PJC] Apocalyptic

WordNet (r) 3.0 (2006):

apocalyptical
    adj 1: prophetic of devastation or ultimate doom [syn:
           apocalyptic, apocalyptical, revelatory]