The bilge is the lowest compartment on a ship where the two sides meet. The word was first coined in 1523.
The word is sometimes also used to describe the water that collects in this compartment. Water that does not drain off the side of the deck drains down through the ship into the bilge. This water may be from rough seas, rain, or minor leaks in the hull or stuffing box. The water that collects in the bilge must be pumped out if it becomes too full and threatens to sink the ship.
Bilge water can be found aboard every vessel. Depending on the ship's design and function, bilge water may contain water, oil, urine, detergents, solvents, chemicals, pitch, particles, and so forth.
By housing water in a compartment, it keeps it beneath the decks, making it safer for the crew to operate the vessel and for people to move around in strong weather.
The development of bilge pumps went from buckets, to hand pumps, and now electric bilge pumps are available for even small vessels. Bilge coatings are applied to protect the surfaces. The water is often noxious, and "bilge water" is a derogatory term often used colloquially to refer to something bad, ruined, or fouled.
A feature of the bilge is the swash plate, which serves to dampen the rush of water from side to side, which might destabilize the ship if allowed to run freely. The swash plates run fore and aft and have small openings near the bottom where the water is allowed to run through.