A leak is a way (usually an opening) for [[fluid]] to escape a container or [[fluid]]-containing system, such as a [[Tank (for storing liquid in)|]] or a ship's [[Hull (watercraft)|]], through which the contents of the container can escape or outside matter can enter the container. Leaks are usually unintended and therefore undesired.
The word "leak" is also used as a verb; matter going through the opening is said to leak. The entry, exit, or exchange of matter through the leak is called , the subject of another disambiguation article.
The matter leaking in or out can be [[gas]], [[liquid]], a highly [[Viscosity|viscous]] paste, or even a [[solid]] such as a [[Powder (substance)|powdered]] or granular solid or other solid particles.
Sometimes the word "leak" is used in a figurative sense. For example, when secret information becomes public, this is known as a [[news leak]].
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Leakage of air or other gas out of [[hot air balloon]]s, [[dirigible]]s, or cabins of [[airplane]]s could present dangerous situations. A leak could even be inside a body, such as a hole in the [[septum]] between [[heart]] ventricles causing an exchange of oxygenated and deoxygenated blood, or a [[fistula]] between bodily cavities such as between [[vagina]] and [[rectum]].
There can be numerous causes of leaks. Leaks can occur from the outset even during [[construction]] or initial manufacture/assembly of fluid systems. [[Pipe (material)|]]s, [[tube (fluid conveyance)|tubing]], [[valve]]s, , or other components may be improperly joined or welded together. Components with threads may be improperly screwed together. Leaks can be caused by damage; for example, punctures or fracture. Often leaks are the result of deterioration of materials from wear or aging, such as rusting or other corrosion or decomposition of [[elastomer]]s or similar [[polymer]] materials used as [[gasket]]s or other [[Seal (mechanical)|seals]]. For example, wearing out of [[faucet]] [[washer (hardware)|]]s causes water to leak at the faucets. Cracks may result from either outright damage, or wearing out by stress such as [[fatigue (material)|fatigue failure]] or corrosion such as [[stress corrosion cracking]]. Wearing out of a surface between a disk and its seat in a valve could cause a leak between ports (valve inlets or outlets). Wearing out of packing around a turning valve stem or rotating [[centrifugal pump]] shaft could develop into fluid outleakage into the environment. For some frequently operating centrifugal pumps, such leakage is so expected that provisions are made for carrying away the leakage. Similarly, wearing out of seals or packing around piston-driven pumps could also develop into outleakage to the environment.