A low-lying area which is wholly or largely surrounded by higher land. An example is Hudson Bay in northeastern Canada, which was formed by depression beneath the center of a continental ice sheet 18,000 years ago. Another example, the Qattara depression, is 150 mi (240 km) long and the largest of several wind-excavated basins of northern Egypt. Depressions in the ocean floor are also basins, such as the Canary Basin, west of northern Africa, or the Argentine Basin, east of Argentina. These basins occur in regions where cold, dense oceanic crust lies between the topographically elevated ocean ridges and the continental margins.Continental margin Marine geology

A drainage basin is the entire area drained by a river and its tributaries. Thus, the Mississippi Basin occupies most of the United States between the Rocky Mountains and the Appalachians. Interior drainage basins consist of depressions that drain entirely inward, without outlet to the sea. Examples may be quite small, such as the Salton Sea of southern California or the Dead Sea of central Asia. One of the most remarkable examples of an interior drainage basin is the Chad Basin in northern Africa, the center of which is occupied by Lake Chad. The fresh waters of the lake drain underground to feed oases in the lowlands 450 mi (720 km) to the northeast.

In the geologic sense, a basin is an area in which the continental crust has subsided and the depression has been filled with sediments. Such basins were interior drainage basins at the time of sediment deposition but need not be so today. As these basins subside, the layers of sediment are tilted toward the axis of maximum subsidence. Consequently, when the sedimentary layers are observed in cross section, their geometry is a record of the subsidence of the basin through time and contains clues about the origin of the basin.

The origin of geologic basins is a topic of continuing interest in both applied and basic geological studies. They contain most of the world's hydrocarbon reserves, and they are regarded as some of the best natural laboratories in which to understand the thermal and mechanical processes that operate deep in the interior of the Earth and that shape the Earth's surface.