1. The plants and animals attached to, living on, in or near the sea bed, river bed, or lake floor: The benthos living in such a region include the permanently attached or immobile forms (e.g., sponges, corals, oysters), creeping forms (e.g., crabs, snails), and the burrowing animals (e.g., worms).

Barnacles, the larger seaweeds, and sea squirts are also members of this group.

The benthos are divided into "sessile" organisms (those that are attached to the bottom or to objects on or near the bottom) and "vagrant" organisms (those that crawl or swim along the bottom).
2. The biogeographic region at the bottom of a sea or ocean (or, more broadly, at the soil-water interface of an ocean, sea, or lake): The benthos must provide the type of living conditions that are necessary for certain plants to grow successfully.

For more information, see Benthos Addendum.

Getting to the bottom of this unit about deep, depth; the fauna and flora of the bottom of the sea.

There is also a separate explanation about the importance of benthos for the earth.