- An airtight chamber such as that used in tunnel and caisson excavation.
- A system of double doorways permitting entry and exit while preventing airflow from one area to another, as from a contaminated area to an uncontaminated area.
- An entrance room between areas of different pressures, such as the entrance to an air-supported structure.
- In plumbing, air trapped in a system and preventing flow.
- noun an obstruction to the flow of a liquid in a pipe, caused by a bubble of air