In general, the word seal refers to elements or structures that prevent the unwanted exchange of substances from one place to another. Some examples are the seals in a tyre valve or a water tap that can be shut off.
A distinction between static and dynamic seals can be made in fluid transport. Static seals cause no relative movement between the parts involved, while dynamic seals move close to each other. Static seals include fibre seals and o-rings. An example of a dynamic seal is the mechanical seal, used to seal a rotating shaft against a fixed wall. In pump systems, seals are used to prevent leakage. Even if they do not ensure complete tightness, they are called a seal, such as in a throttle gap. In addition, seals are also required in liquid handling outside the pump units, for example, in fittings and pipe connections.