The Collaborative International Dictionary of English v.0.48:

Abyssal \A*byss"al\, a. [Cf. Abysmal.]
   Belonging to, or resembling, an abyss; unfathomable.
   [1913 Webster]

   Abyssal zone (Phys. Geog.), one of the belts or zones into
      which Sir E. Forbes divides the bottom of the sea in
      describing its plants, animals, etc. It is the one
      furthest from the shore, embracing all beyond one hundred
      fathoms deep. Hence, abyssal animals, plants, etc.
      [1913 Webster]

WordNet (r) 3.0 (2006):

abyssal
    adj 1: relating to ocean depths from 2000 to 5000 meters
    2: resembling an abyss in depth; so deep as to be unmeasurable;
       "the abyssal depths of the ocean" [syn: abysmal, abyssal,
       unfathomable]