The deep sea represents those marine environments that occur beyond the continental shelf. The average depth at which this occurs is approximately 200m, so typically we define the as those environments greater than 200 meters and extending to approximately 10,000 m (the depth of Mariana’s Trench, the deepest point in the ocean).
Although the defining feature is not depth per se but rather the transition from the continental shelf to the continental slope, referred to as the shelf break. The term deep sea can be used to characterize both deep pelagic habitats, the water column greater than 200m, and deep benthic habitats, the seafloor greater than 200m. The slope is the region beyond the shelf where the seafloor rapidly increases in depth (~200-2000m although these estimates rely solely on the topography of the region). From 2000-4000m (again approximate) the seafloor’s depth increases more gradually and this section is denoted as the continental rise. Both the slope and rise are referred to as the bathyal region. Eventually at great depths, very little change occurs in depth and these large extents of the bottom are referred to as the abyss or abyssal plains. Of course all of these areas can be perforated by trenches with depths up to 10,000m.