a·byss·al / əˈbisəl/ • adj. chiefly technical relating to or denoting the depths or bed of the ocean, esp. between about 10,000 and 20,000 feet (3,000 and 6,000 m) down. ∎  Geol. another term for plutonic (sense 1).

abyssal Applied to the deepest part of the ocean, below about 2000 m. The abyssal zone lies seaward of, and deeper than, the bathyal zone, and covers approximately 75 per cent of the ocean floor. It is the most extensive Earth environment, cold, dark, with slow-moving currents (less than a few centimetres per second), supporting fauna that are typically black or grey, delicately structured, and not streamlined. Compare neritic.

abyssal Term to describe oceanic features occurring at great depths, usually more than c.3000m (10,000ft) below sea level. Abyssal plains cover c.30% of the Atlantic and nearly 75% of the Pacific ocean floors. They are covered by deposits of biogenic oozes formed by the remains of microscopic plankton and nonbiogenic sediments. The gradient is less than 1:1000, except for the occasional low hill. The abyssal zone is the deepest area of the ocean. It receives no sunlight, so there are no seasons and no plants, but there are many forms of life, such as sponges, crinoids (sea lilies) and brachiopods (lampshells).

abyssal Applied to the deepest part of the ocean, below about 2000 m. The abyssal zone covers approximately 75% of the ocean floor. Compare BATHYAL; NERITIC.