Data: Information, usually expressed as numbers. ("Data" is actually a plural noun. The singular is "datum." ) Data are often used in analysis to draw conclusions that support or reject a hypothesis.

Debris flows: Relatively slow movement of sediments downward over gently sloping terrain.

Decomposer: An organism, like a bacterium, that causes the decay of dead plant and animal matter.

Deep scattering layer: A concentrated layer of midwater organisms that can reflect and scatter sound waves produced by sonar devices.

Deep sea: The deep, lower regions of the ocean where sunlight doesn't penetrate.

Delaminate: Process by which layers of material come apart by action of salt water and air.

Delegate: (noun) a representative who speaks for a group of people at a meeting or lawmaking assembly. (verb) To give someone authority or responsibility to carry out a task.

Deck: The covered part of the top of a boat.

Decomposer: Organism that breaks down dead tissues, releasing simple chemical substances (minerals) into the environment

Defend: To protect - refers to a territory.

Degrease: Remove grease from engines.

Density: The mass per unit volume of a substance or the number of individuals per unit area.

Deposit feeder: An animal that eats organic matter that's been deposited on or in the seafloor.

Depth sounder: An electronic device that measures the depth of water very precisely.

Desiccation: The loss of water from an organism due to drying in the air or the osmotic stress of a high-salinity environment.

Device: An instrument, tool or machine designed and constructed to perform a certain task.

Diaphragm: A vibrating membrane or disc used in an aquarium pump to send air in one direction.

Diatoms: Microscopic single-celled algae which have two ornate interfitting outer 'shells' containing silica.

Digital display: Display of numbers on an instrument.

Dinghy: A small boat used as a tender for a larger boat.

Disintegrate: To break up into smaller pieces.

Display: A set of calls or motions done by a bird or animal to convey a particular message. Penguins might do a courtship display to attract a mate, or an aggressive display to warn off an enemy.

Dissolved oxygen bottle: Small bottle used in determining oxygen concentration of a water sample.

Distribution: The range over which a species lives.

Diurnal: daily.

Diving bell: Vessel with compressed air used in deep sea diving to hold one or more people.

Dogfish: A species of small shark.

Doliolid: A deep sea pelagic tunicate.

Dolphin: A member of a group of small whales that have sharp, pointed teeth and pointed snouts.

Dorsal: On or toward the back or topside (opposite of ventral).

Dragging: Another term for bottom trawling, a fishing method in which large nets are dragged along the seafloor to catch fish.

Drain spraying: To paint a sign with a stencil on a drain to increase public awareness about stormwater pollution.

Dreamtime: Time in Aboriginal spiritual culture during which the Earth received its present form and in which the patterns and cycles of life and nature were initiated.

Dredge: A net attached to a frame that's dragged along the ocean floor to collect animals.

Drift kelp: A piece of seaweed that breaks free of its holdfast, drifts with the ocean currents and sinks to the seafloor.

Drop in: To catch a wave in front of another surfer who is already on the wave. Dangerous practice often leading to surfing injuries.

Duck diving: To push your board under a broken wave so as to avoid the break.

Dune: A hill of sand. The wind pushes sand from the beach into dunes.