A sump is a low reservoir that collects often undesirable liquids such as water or chemicals. A sump can also be an infiltration basin used to manage surface runoff water and recharge underground aquifers.^[1] Sump can also refer to an area in a cave where an underground flow of water exits the cave into the earth.

The lowest point in a basement collects water that enters from outside. If this is a regular occurrence, a sump pump can move the water outside of the house.

Another example is the oil pan of an engine. Oil pools in a reservoir known as its sump, at the bottom of the engine. Use of a sump requires the engine to be mounted slightly higher to make space for it. Often though, oil in the sump can slosh during hard cornering, starving the oil pump. For these reasons, racing motorcycles and piston aircraft engines are "dry sumped" using scavenge pumps and a swirl tank to separate oil from air, which is sucked up by the pumps.^[2]

Aquariums can also include a sump, mainly a reef system. The sump sits below the main tank and also acts as a filter, as well as holding unsightly equipment such as heaters and protein skimmers. The main advantage of plumbing a sump into an aquarium is the increase of water in the system, making it more stable and less prone to fluctuations of pH and salinity.

A diving snorkel can have a sump section located below the mouthpiece. This allows excess moisture from the breath and liquid from the ocean to settle and remain in the sump, so that it does not impair breathing.

In a nuclear power plant's reactor housing, the sump collects any overflow of primary loop coolant; in this case, monitoring and pumping of the sump is an important part of the reactor's safety system.

In mining the sump is a hole in the floor of a level in a working, in the direction of a lower level either to test the trend of an ore vein, or for ventilation.

The equivalent of a sump on a boat is the bilge.

In the human eye, the vitreous humour has a minor role as a metabolic sump.^[3]

In caving/potholing a sump is a permanently flooded section of a cave, such that the caver must submerge under water to reach the other side.

In a foxhole, a grenade sump is a deeper hole dug inside the foxhole into which live grenades can be kicked to minimize damage from the explosion.

In medieval cosmology, the sump was the center of the cosmos, where the dregs and filth descended, with the celestial sphere far exalted above the world of fallen man.

• Sump pump – Type of pump