[1] : The Collaborative International Dictionary of English v.0.48

abyssal \abyss"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
see also:
abysmal abyssal zone 

[2] : WordNet (r) 2.0

abyssal
     adj 1: relating to ocean depths from 2000 to 5000 meters
     2: so deep as to be unmeasurable; "the abyssal depths of the
        ocean" syn: abysmal, unfathomable
see also:
abysmal unfathomable 

Abyssal >> A Words
Abyssal, definition of term: Abyssal
abyssal_pag1.html

Abyssal Zone >> A Words
Abyssal Zone, definition of term: Abyssal Zone
abyssal+zone_pag1.html