Relating to or occurring in the depths of the ocean, typically between about 3,000 and 6,000 metres (10,000–20,000 feet). The abyssal creatures have adapted to survive in the extreme pressures of the deep sea.
Immeasurably deep or profound. His abyssal ignorance of the subject was evident during the discussion.
Geology: relating to or characteristic of igneous rocks formed at great depths, especially those constituting the oceanic crust. Abyssal basalts form a large part of the seafloor beneath mid-ocean ridges.