A sump is a low space that collects any often-undesirable liquids such as water or chemicals.
An example is the oil pan of an engine. The oil is used to lubricate the engine's moving parts and it pools in a reservoir, known as a sump, at the bottom of the engine. Use of a sump requires the engine to be mounted slightly higher to make space for it. Oil in the sump can surge during hard cornering, starving the oil pump; for these reasons, race engines are "dry sumped" using scavenge pumps and a swirl tank to separate oil from air which is also sucked up by the pumps.
One common example of a sump is the lowest point in a basement into which any water that seeps in flows. If this is a regular problem, a sump pump that moves the water outside of the house may be used. If the building is a nuclear power plant's reactor housing, the role of the sump will be to collect 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.
Another example is in an aquarium, mainly a reef system. The sump sits below the main tank and is used as a filter, as well as a holding place of unsightly equipment such as heaters and protein skimmers. The main advantage of having a sump plumbed 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 the snorkeler's breathing.
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.
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.