A leak is a hole or other opening, usually unintended and therefore undesired, in a container or fluid-containing system, such as a tank or a ship's hull, through which the contents of the container can escape or outside matter can enter the container. 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 leakage, the subject of another disambiguation article. The matter leaking in or out can be gas, liquid, a highly viscous paste, or even a solid such as a powdered or granular solid or other solid particles. A leak can be between two (or more) fluid-containing systems, allowing transfer of matter from either system to the other, or even an exchange of matter between them. Leakage of matter into a container or other system could be called inleakage. Leakage of matter out of a container or other system could be called outleakage. The presence of a leak does not necessarily mean there is always leakage of matter; it just implies there could be leakage through the opening. If the container or system is empty in an empty environment, there is no leakage at that time. Sometimes the word "leak" is used to refer to leakage in a certain situation, for example the passing or making public of secret information.

Leak types and possible causes

Types of leak openings include a puncture, gash, rust or other corrosion hole, very tiny pinhole leak (possibly in imperfect welds), crack or microcrack, or inadequate sealing between components or parts joined together. When there is a puncture, the size and shape of the leak can often be seen, but in many other cases, the size and shape of the leak opening may not be so obvious. In many cases, the location of a leak can be determined by seeing material drip out at a certain place, although the leak opening itself is not obvious. In some cases, it may known or suspected there is a leak, but even the location of the leak is not known. Since leak openings are often so irregular, leaks are sometimes sized by the leakage rate, as in volume of fluid leaked per time, rather than the size of the opening.

Common types of leaks for many people include leaks in vehicle tires, causing air to leak out resulting in flat tires, and leaks in containers, spilling the contents. Leaks can occur or develop in many different kinds of household, building, vehicle, marine, aircraft, or industrial fluid systems, whether the fluid is a gas or liquid. Leaks in vehicle hydraulic systems such as brake or power steering lines could cause outleakage of brake or power steering fluid resulting in failure of the brakes, power steering, or other hydraulic system. Also possible are leaks of engine coolant - particularly in the radiator and at the water pump seal, transmission fluid, motor oil, and refrigerant in the air conditioning system. Some of these vehicle fluids have different colors to help identify the type of leaking fluid.