A leak is a way (usually an opening) for fluid to escape 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. Leaks are usually unintended and therefore undesired. The word leak usually refers to a gradual loss; a sudden loss is usually called a spill.
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.
Sometimes the word "leak" is used in a figurative sense. For example, in a news leak secret information becomes public.
According to ASTM D7053-17, water leakage is the passage of (liquid) water through a material or system designed to prevent passage of water.
As an indication, the market for sealants (to seal buildings and fittings against leakage) in the UK in 1986 was £18 million.